drm/amdgpu: drop support for untouched registers

I couldn't figure out what this was original good for, but we
don't use it any more.

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