drm/amdgpu: skip CG for VCN when late_init/fini

VCN clockgating is handled manually like VCE and UVD.

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