drm/amdgpu: skip ASD fw load for sienna_cichlid

Skip ASD FW load for sienna_cichlid currently.

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