I've just tried newest drm-next-4.9-wip:        
1c22b05623e5e03ada5a767951eac3203b246be9

and there is something new in kernel log:

[ 3430.379659] amdgpu 0000:01:00.0: fb1: amdgpudrmfb frame buffer device
[ 3431.438851] [drm:gfx_v6_0_ring_test_ib] *ERROR* amdgpu: IB test timed out    
[ 3431.438862] [drm:amdgpu_ib_ring_tests] *ERROR* amdgpu: failed testing IB on
GFX ring (-110).
[ 3431.438866] [drm:amdgpu_device_init] *ERROR* ib ring test failed (-110).     
[ 3431.871374] [drm:amdgpu_late_init] *ERROR* late_init of IP block
<amdgpu_powerplay> failed -22
[ 3431.871381] amdgpu 0000:01:00.0: amdgpu_late_init failed                     
[ 3431.871386] amdgpu 0000:01:00.0: Fatal error during GPU init                 
[ 3431.871390] [drm] amdgpu: finishing device.

after that, sysrq works only (and ssh).