drm/amdgpu: Skip using MC FB Offset when APU flag is set for SRIOV.

The MC_VM_FB_OFFSET is PF only register. It cannot be read on VF.
So, the driver should not use MC_VM_FB_OFFSET address to set the
address of dev->gmc.aper_base.

Signed-off-by: Gavin Wan <Gavin.Wan@amd.com>
Reviewed-by: Zhigang Luo <zhigang.luo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>