drm/virtio: drop virtio_gpu_fence_cleanup()

Just call drm_fence_put directly instead.
Also set vgfb->fence to NULL after dropping the reference.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20181219122708.4586-4-kraxel@redhat.com