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

KMD should not program these registers, the value were
defined in the host, so skip them in the SRIOV environment.

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