video: imxfb: Remove unused fields from platform data structure

Some fields in platform data structure is never used by boards.
This patch removes these fields and as a result optimizes private
driver structure a bit. Additionally patch removes backligh_power()
callback, so if it will be needed in the future, this feature should
be added as pwm{gpio,etc.}-regulator to the board code or in the DTS.

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>