drm/nouveau: when bailing out of a pushbuf ioctl, do not remove previous fence

commit 9360bd1112d8874d21942e2ae74f5416b00a8db6 upstream.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>