R9 285 Tonga,  testing with amdgpu.dc=0 on agd5f 4.17-wip

Since

commit a02497b73218f10f237d98fb10d34d0baed607a0
Author: Rex Zhu <Rex.Zhu@amd.com>
Date:   Fri Feb 9 16:47:53 2018 +0800

    drm/amd/pp: Fix bug that dpm level was not really locked

    Lock the dpm levels when we use SW method to modify
    the dpm tables directly to avoid a possible race
    with the smu.

weston may hang display on start/quit/vt switch

SysRq works and logged will be errors like -

Mar  2 00:19:03 ph4 kernel: amdgpu: [powerplay] 
Mar  2 00:19:03 ph4 kernel:  failed to send message 18b ret is 0 
Mar  2 00:19:03 ph4 kernel: amdgpu: [powerplay] 
Mar  2 00:19:03 ph4 kernel:  failed to send pre message 18a ret is 0 
Mar  2 00:19:04 ph4 kernel: amdgpu: [powerplay] 
Mar  2 00:19:04 ph4 kernel:  failed to send message 18a ret is 0 
Mar  2 00:19:04 ph4 kernel: amdgpu: [powerplay] 
Mar  2 00:19:04 ph4 kernel:  failed to send pre message 18c ret is 0 
Mar  2 00:19:04 ph4 kernel: amdgpu: [powerplay] 
Mar  2 00:19:04 ph4 kernel:  failed to send message 18c ret is 0 
Mar  2 00:19:05 ph4 kernel: amdgpu: [powerplay] 
Mar  2 00:19:05 ph4 kernel:  failed to send pre message 145 ret is 0 
Mar  2 00:19:05 ph4 kernel: amdgpu: [powerplay] 
Mar  2 00:19:05 ph4 kernel:  failed to send message 145 ret is 0 
Mar  2 00:19:06 ph4 kernel: amdgpu: [powerplay] 
Mar  2 00:19:06 ph4 kernel:  failed to send pre message 146 ret is 0 
Mar  2 00:19:06 ph4 kernel: amdgpu: [powerplay] 
Mar  2 00:19:06 ph4 kernel:  failed to send message 146 ret is 0 
Mar  2 00:19:07 ph4 kernel: amdgpu: [powerplay] 
Mar  2 00:19:07 ph4 kernel:  failed to send pre message 148 ret is 0