drm/amdgpu: deal with foreign fences in amdgpu_sync

This also requires some error handling from the callers of that function.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>