video: imxfb: Use regulator API with LCD class for powering

This patch replaces custom lcd_power() callback with
regulator API over LCD class.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>