drm/amdgpu/vcn3.0: schedule instance 0 for decode and 1 for encode

VCN3 has 2 unsymmetrical instances, i.e there're less codecs
on instance 1, we use 0 for decode and 1 for encode for now

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