OMAPDSS: add __exit to dss_uninit_ports

dss_uninit_ports is only called by __exit omap_dsshw_remove

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: linux-omap@vger.kernel.org
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>