OMAP: DSS2: OMAPFB: Skip unnecessary set_overlay_info()

In omapfb_enable_overlay() if the overlay state is already what we want
skip the set_overlay_info().

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