drm/amdgpu:cg & pg shouldn't active on VF device

CG & PG function changes engine clock/gating, which is
not appropriate for VF device, because one vf doesn't know
the whole picture of engine's overall workload.

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>