video: imxfb: Select LCD_CLASS_DEVICE unconditionally

FB driver uses lowlevel controls for LCD powering and contrast changing.
Since LCD class cannot be used as an optional feature and should be
compiled for using in the driver, this patch selects LCD_CLASS_DEVICE
symbol for the driver.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>