drm/amdgpu: implement PRT for GFX7 v2

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

v2: set correct register

Signed-off-by: Junwei Zhang <Jerry.Zhang@amd.com>
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>