drm/qxl: unset a pointer in sync_obj_unref

commit 41ccec352f3c823931a7d9d2a9c7880c14d7415a upstream.

This fixes a BUG_ON(bo->sync_obj != NULL); in ttm_bo_release_list.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>