drm/amd/sriov skip vcn powergating and dec_ring_test

1.Skip decode_ring test in VF, because VCN in SRIOV does not
support direct register read/write.

2.Skip powergating configuration in hw fini because
VCN3.0 SRIOV doesn't support powergating.

V2: delete unneccessary white lines and refine implementation.

Signed-off-by: Jack Zhang <Jack.Zhang1@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>