OMAP: DSS2: OMAPFB: Remove FB_OMAP2_FORCE_AUTO_UPDATE

Remove the option for forcing auto-update. Auto-update for manual update
displays is no more a DSS feature, so if a particular display devices does
have auto-update mode, it should be in display's custom settings.

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