drm/amdgpu: clarify UVD/VCE special handling for CG

UVD and VCE CG are handled specially, however the previous
fix for this skipped late init for those blocks rather than
just CG.  Just protect the CG function call.  No functional
change since UVD and VCE don't currently utilize a late_init
function.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>