drm/amdgpu/vcn: Skip vcn power-gating change for sriov

CG/PG is handled on the host side.

Signed-off-by: Samir Dhume <samir.dhume@amd.com>
Acked-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>