Hello there,
I am seeing the same problems on my Ryzen 2700U which is freezing as well, with
the latest Ubuntu kernel: 4.19.0-041900-generic.

I am luckily able to ssh back in and try to shut it down, but it won't
completely.

This is what I see on the kern.log, and in my case the whole graphics is
frozen.
I am running linuxmint cinnamon 19, with the latest Ubuntu kernel.

Oct 28 11:30:58 antonioRyzen kernel: [22639.758782] gmc_v9_0_process_interrupt:
10 callbacks suppressed
Oct 28 11:30:58 antonioRyzen kernel: [22639.758789] amdgpu 0000:02:00.0:
[gfxhub] VMC page fault (src_id:0 ring:24 vmid:7 pasid:32769, for p
rocess cinnamon pid 1459 thread amdgpu_cs:0 pid 1463
Oct 28 11:30:58 antonioRyzen kernel: [22639.758789] )
Oct 28 11:30:58 antonioRyzen kernel: [22639.758797] amdgpu 0000:02:00.0:   at
address 0x00000001010e1000 from 27
Oct 28 11:30:58 antonioRyzen kernel: [22639.758801] amdgpu 0000:02:00.0:
VM_L2_PROTECTION_FAULT_STATUS:0x00701031
Oct 28 11:30:58 antonioRyzen kernel: [22639.758818] amdgpu 0000:02:00.0:
[gfxhub] VMC page fault (src_id:0 ring:24 vmid:7 pasid:32769, for process
cinnamon pid 1459 thread amdgpu_cs:0 pid 1463
Oct 28 11:30:58 antonioRyzen kernel: [22639.758818] )
Oct 28 11:30:58 antonioRyzen kernel: [22639.758822] amdgpu 0000:02:00.0:   at
address 0x00000001010e0000 from 27
Oct 28 11:30:58 antonioRyzen kernel: [22639.758825] amdgpu 0000:02:00.0:
VM_L2_PROTECTION_FAULT_STATUS:0x00000000
Oct 28 11:30:58 antonioRyzen kernel: [22639.758834] amdgpu 0000:02:00.0:
[gfxhub] VMC page fault (src_id:0 ring:24 vmid:7 pasid:32769, for process
cinnamon pid 1459 thread amdgpu_cs:0 pid 1463
Oct 28 11:30:58 antonioRyzen kernel: [22639.758834] )
Oct 28 11:30:58 antonioRyzen kernel: [22639.758839] amdgpu 0000:02:00.0:   at
address 0x00000001010e0000 from 27
Oct 28 11:30:58 antonioRyzen kernel: [22639.758841] amdgpu 0000:02:00.0:
VM_L2_PROTECTION_FAULT_STATUS:0x00000000
Oct 28 11:30:58 antonioRyzen kernel: [22639.758850] amdgpu 0000:02:00.0:
[gfxhub] VMC page fault (src_id:0 ring:24 vmid:7 pasid:32769, for process
cinnamon pid 1459 thread amdgpu_cs:0 pid 1463
Oct 28 11:30:58 antonioRyzen kernel: [22639.758850] )
Oct 28 11:30:58 antonioRyzen kernel: [22639.758853] amdgpu 0000:02:00.0:   at
address 0x00000001010e0000 from 27
Oct 28 11:30:58 antonioRyzen kernel: [22639.758855] amdgpu 0000:02:00.0:
VM_L2_PROTECTION_FAULT_STATUS:0x00000000
Oct 28 11:30:58 antonioRyzen kernel: [22639.758863] amdgpu 0000:02:00.0:
[gfxhub] VMC page fault (src_id:0 ring:24 vmid:7 pasid:32769, for process
cinnamon pid 1459 thread amdgpu_cs:0 pid 1463
Oct 28 11:30:58 antonioRyzen kernel: [22639.758863] )
Oct 28 11:30:58 antonioRyzen kernel: [22639.758867] amdgpu 0000:02:00.0:   at
address 0x00000001010e0000 from 27
Oct 28 11:30:58 antonioRyzen kernel: [22639.758869] amdgpu 0000:02:00.0:
VM_L2_PROTECTION_FAULT_STATUS:0x00000000
Oct 28 11:30:58 antonioRyzen kernel: [22639.758877] amdgpu 0000:02:00.0:
[gfxhub] VMC page fault (src_id:0 ring:24 vmid:7 pasid:32769, for process
cinnamon pid 1459 thread amdgpu_cs:0 pid 1463
...
Oct 28 11:30:58 antonioRyzen kernel: [22639.758931] )
Oct 28 11:30:58 antonioRyzen kernel: [22639.758935] amdgpu 0000:02:00.0:   at
address 0x00000001010e1000 from 27
Oct 28 11:30:58 antonioRyzen kernel: [22639.758937] amdgpu 0000:02:00.0:
VM_L2_PROTECTION_FAULT_STATUS:0x00000000
Oct 28 11:31:08 antonioRyzen kernel: [22649.811916] [drm:amdgpu_job_timedout
[amdgpu]] *ERROR* ring gfx timeout, signaled seq=1445040, emitt
ed seq=1445042
Oct 28 11:31:08 antonioRyzen kernel: [22649.811925] [drm] GPU recovery
disabled.
Oct 28 11:33:58 antonioRyzen kernel: [22820.391906] INFO: task
kworker/u32:1:19683 blocked for more than 120 seconds.
Oct 28 11:33:58 antonioRyzen kernel: [22820.391914]       Not tainted
4.19.0-041900-generic #201810221809
Oct 28 11:33:58 antonioRyzen kernel: [22820.391917] "echo 0 >
/proc/sys/kernel/hung_task_timeout_secs" disables this message.
Oct 28 11:33:58 antonioRyzen kernel: [22820.391920] kworker/u32:1   D    0
19683      2 0x80000000
Oct 28 11:33:58 antonioRyzen kernel: [22820.391943] Workqueue: events_unbound
commit_work [drm_kms_helper]
Oct 28 11:33:58 antonioRyzen kernel: [22820.391945] Call Trace:
Oct 28 11:33:58 antonioRyzen kernel: [22820.391956]  __schedule+0x29e/0x840
Oct 28 11:33:58 antonioRyzen kernel: [22820.391959]  schedule+0x2c/0x80
Oct 28 11:33:58 antonioRyzen kernel: [22820.391962] 
schedule_timeout+0x258/0x360
Oct 28 11:33:58 antonioRyzen kernel: [22820.392050]  ?
optc1_get_crtc_scanoutpos+0x69/0xa0 [amdgpu]
Oct 28 11:33:58 antonioRyzen kernel: [22820.392062] 
dma_fence_default_wait+0x20a/0x280
Oct 28 11:33:58 antonioRyzen kernel: [22820.392065]  ?
dma_fence_release+0xa0/0xa0
Oct 28 11:33:58 antonioRyzen kernel: [22820.392068] 
dma_fence_wait_timeout+0xe7/0x110
Oct 28 11:33:58 antonioRyzen kernel: [22820.392071] 
reservation_object_wait_timeout_rcu+0x201/0x340
Oct 28 11:33:58 antonioRyzen kernel: [22820.392140]  ?
amdgpu_get_vblank_counter_kms+0x111/0x160 [amdgpu]
Oct 28 11:33:58 antonioRyzen kernel: [22820.392222] 
amdgpu_dm_do_flip+0x12c/0x370 [amdgpu]
Oct 28 11:33:58 antonioRyzen kernel: [22820.392305] 
amdgpu_dm_atomic_commit_tail+0x7ac/0xea0 [amdgpu]