OMAPFB: disable overlays on driver removal

When omapfb module is removed, the driver will turn off all the displays
it manages. However, it leaves the overlays enabled. While the overlays
are obviously disabled as the displays are disabled, it causes issues
when the driver module is loaded again, as at that point the overlays
are still enabled on the hardware level. The result is that even if the
SW thinks overlays are disabled, they are actually enabled.

Fix this by making sure the overlays are disabled at module removal.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>