staging: xgifb: disable LVDS on XG27

XG27 has the second display output already hardcoded to disabled. Just
in case, ensure that it has the LVDS code paths disabled. This will
simplify the future cleanups.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>