UPSTREAM: 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


(cherry picked from commit cb66c6daa5e9b2b2aa7ebe956e329c81c1ffe466)
Signed-off-by: Greg Hartman <ghartman@google.com>
BUG: 139386237
Change-Id: I57ec957d37733db30aeb4b329415a1f81f59936f