drm/amdgpu: Skip the VRAM base offset on SRIOV

[Why]
As VF cannot read MMMC_VM_FB_OFFSET with L1 Policy(read 0xffffffff).
It leads to driver get the incorrect vram base offset.

[How]
Since SR-IOV is dGPU only, skip reading this register and set the
fb_offest to 0.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Yifan Zha <Yifan.Zha@amd.com>
Signed-off-by: Horace Chen <horace.chen@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>