backlight: Fix order of Kconfig entries

Switch the order of LCD_CLASS_DEVICE and BACKLIGHT_CLASS_DEVICE,
so that it's possible to insert LCD devices without borking the
dependency displays of xconfig and other config tools.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Richard Purdie <rpurdie@rpsys.net>