drm/amdgpu: fix NULL fence handling in amdgpu_cs_fence_to_handle_ioctl

When the fence is already signaled it is perfectly normal to get a NULL
fence here. But since we can't export that we need to use a stub fence.

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