Created attachment 142294 [details]
dmesg spew

With radeon.cik_support=0 and amdgpu.cik_support=1, I get a blank screen on
Linux 4.19.0. This worked fine on 4.18.16. Machine is still accessible via SSH,
but will not shut down successfully.

Using radeon instead of amdgpu is a work-around.

By the function names I'm guessing it's UVD/PowerPlay related, but this is out
of my realm of troubleshooting expertise. 

Attached dmesg snippet with traces and detailed error messages.