drm/amdgpu: Skip some registers config for SRIOV

Some registers are not accessible to virtual function setup, so
skip their initialization when in VF-SRIOV mode.

v2: move SRIOV VF check into specify functions;
modify commit description and comment.

Signed-off-by: Liu ChengZhe <ChengZhe.Liu@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>