drm/amdgpu/vcn: add vcn multiple AIDs support

add vcn multiple AIDs support.

v2: squash in FW setting fix (Alex)

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