OMAP: DSS2: Check if display supports update mode changes

Check whether the display actually has the set_update_mode() function
before calling it. Only the sysfs codepath was broken, the omapfb ioctl
had the necessary protection.

Signed-off-by: Ville Syrjälä <ville.syrjala@nokia.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>