drm/amdgpu: wire up emit_wreg for gfx v6

Needed for vm_flush unification.

v2: handle compute rings as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>