drm/omap: add framedone interrupt support

This prepares framedone interrupt handling for
manual display update support.

Acked-by: Pavel Machek <pavel@ucw.cz>
Tested-by: Tony Lindgren <tony@atomide.com>
Tested-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>