drm/amd/amdgpu/vcn: Add RB decouple feature under SRIOV - P2

- Add function to check if RB decouple is enabled under SRIOV

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