drm/virtio: fix command submission with objects but without fence.

Only call virtio_gpu_array_add_fence if we actually have a fence.

Fixes: da758d51

 ("drm/virtio: rework virtio_gpu_execbuffer_ioctl fencing")
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190904074828.32502-1-kraxel@redhat.com