The same problem on my machine:

# dmesg | grep -i drm
[   11.753729] [drm] radeon kernel modesetting enabled.
[   11.760535] [drm] initializing kernel modesetting (CEDAR 0x1002:0x68F9
0x1787:0x2291 0x00).
[   11.767360] [drm] register mmio base: 0x80140000
[   11.770683] [drm] register mmio size: 131072
[   11.899433] [drm] GPU not posted. posting now...
[   11.903067] [drm] Detected VRAM RAM=512M, BAR=256M
[   11.903068] [drm] RAM width 64bits DDR
[   11.934284] [drm] radeon: 512M of VRAM memory ready
[   11.936547] [drm] radeon: 1024M of GTT memory ready.
[   11.938855] [drm] Loading CEDAR Microcode
[   13.060828] [drm] Internal thermal controller with fan control
[   13.075872] [drm] radeon: dpm initialized
[   13.099787] [drm] GART: num cpu pages 262144, num gpu pages 262144
[   13.151601] [drm] PCIE GART of 1024M enabled (table at 0x000000000014C000).
[   13.166212] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[   13.166214] [drm] Driver supports precise vblank timestamp query.
[   13.166323] [drm] radeon: irq initialized.
[   13.377755] [drm:0xd000000001d22d04] *ERROR* radeon: ring 0 test failed
(scratch(0x8504)=0xCAFEDEAD)

# inxi -b
System:    Host: T801 Kernel: 4.11.1-gentoo ppc64 (32 bit) Console: tty 0
Distro: Gentoo Base System release 2.3
Machine:   No /sys/class/dmi; using dmidecode: dmidecode is not installed.      
CPU:       Dual core PPC970MP altivec supported (-MCP-) speed/max:
2300.000000MHz/2300 MHz                                                         
Graphics:  Card-1: NVIDIA NV43 [GeForce 6600]                                   
           Card-2: Advanced Micro Devices [AMD/ATI] Cedar [Radeon HD
5000/6000/7350/8350 Series]                                                     
           Display Server: X.org 1.19.3 drivers: fbdev,ati,radeon,nouveau       
           tty size: 211x52 Advanced Data: N/A for root out of X                

But I can confirm the ring test succeeds on kernel 4.10.16. I will try to
bisect starting from the 4.11-rcs.