OMAPDSS: gracefully disable overlay at error

Disable overlay via ovl->disable() interface, which will
properly set flags in cache and GO bits for managers.
This allows overlay user to re-enable it on next frame,
thus recovering from FIFO underflows.

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