drm/amdgpu: remove gfx8 registers that vary between asics

those register mask definitions are different in polaris compare to
former gfx 8 gpus, so remove them from misusing.

Signed-off-by: Ken Wang <Qingqing.Wang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>