drm/qxl: unset a pointer in sync_obj_unref

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

Cc: stable@vger.kernel.org #v3.10+

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>