OMAPDSS: acx565akm: remove platform backlight calls

acx565akm has support to call set_backlight/get_backlight in platform
code. They are not used by any board, and thus can be removed from the
driver.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Archit Taneja <archit@ti.com>