drm/amdgpu/vcn: support unified queue only in vcn4

- remove multiple queue support
- add unified queue related functions
Acked-by: NLeo Liu <leo.liu@amd.com>
Signed-off-by: NRuijing Dong <ruijing.dong@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>