regression on raven (gmc9)

[Tom St Denis]

This commit

[root at raven linux]# git bisect bad
c704d7a15d292e6e222b7e990ed7e9f41617cdd8 is the first bad commit
commit c704d7a15d292e6e222b7e990ed7e9f41617cdd8
Author: Alex Deucher <alexander.deucher at amd.com>
Date:   Tue Mar 27 17:10:56 2018 -0500

     drm/amdgpu/gmc9: use amdgpu_ring_emit_reg_write_reg_wait in gpu tlb 
flush

     Use amdgpu_ring_emit_reg_write_reg_wait.  On engines that support it,
     it provides a write and wait in a single packet which avoids a missed
     ack if a world switch happens between the request and waiting for the
     ack.

     Reviewed-by: Huang Rui <ray.huang at amd.com>
     Reviewed-by: Christian König <christian.koenig at amd.com>
     Signed-off-by: Alex Deucher <alexander.deucher at amd.com>

:040000 040000 a08a12a316d09ea231337136cc7aa076d4da44a8 
521c8a18b0c63e185e416a267619a92822d7f268 M      drivers

Causes my raven1 to lockup on starting X with a gfx ring stall.

Tom