drm/amdgpu: Skip the program of GRBM_CAM* in SRIOV

KMD should not the program these registers,
so skip them in the SRIOV environment.

Signed-off-by: Peng Ju Zhou <PengJu.Zhou@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>