Comment # 21 on bug 108096 from Andrey Grodzovsky

Please load the driver in debug mode so I can see the error code value in dmesg
- 
when loading the kernel add drm.debug=0xff

Also to trace where exactly the error originated from please install trace-cmd
and beore starting X (assuming you get the failure and the dmesg error right on
start)
sudo trace-cmd start -p function_graph -l amdgpu_cs_ioctl
and get the output from /sys/kernel/debug/tracing/trace