I tried with drm-next-5.2-wip.

It does not hang any more, but I have a new error now.

It is better, in the sense that I can now reboot the system normally, and not
resort to echo b >/proc/sysrq-trigger

[drm:amdgpu_cs_ioctl [amdgpu]] *ERROR* Failed to initialize parser -125!

After the GPU reset, the screen is corrupted.
I can do, via ssh,  service gdm stop.  service gdm start   and I then get a
working login screen. (Mouse moves, I can type in password)
I cannot actually login because X fails. The desktop fails to appear and it
returns to the login greeter screen.

I will try to get more details when I have time later.