drm/amdgpu: implement PRT for GFX6 v2

Enable/disable the handling globally for now and
print a warning when we enable it for the first time.

v2: write to the correct register, adjust bits to that hw generation
v3: fix compilation, add the missing register bit definitions

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>