video: imxfb: Resolve mismatch between backlight/contrast

Currently, driver uses backlight class to control contrast value.
This is not correct. This patch resolves this issue by removing
backlight class from the driver and replace handling of LCDC PWM
Contrast Control Register by contrast control through LCD class.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>