drm/amdgpu: retire RREG32_IDX/WREG32_IDX

those are not needed anymore

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