drm/amdgpu/vcn3.0: only SIENNA_CICHLID need specify instance for dec/enc

Only SIENNA_CICHLID(VCN3) has 2 unsymmetrical instances, there're less
codecs on instance 1, we use 0 for decode and 1 for encode.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>