drm/amdgpu/vcn: Add firmware share memory support

Added firmware share memory support for VCN. Current multiple
queue mode is enabled only.

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>