backlight: lp855x: set zero brightness at FBBLANK

When backlight turns on early from display, a white line can be
seen on the screen. Therefore make sure backlight is off when we
are under an fb blank event.

Signed-off-by: Shingo Nakao <shingo.x.nakao@sonymobile.com>
Cc: Milo Kim <milo.kim@ti.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Oskar Andero <oskar.andero@sonymobile.com>
Acked-by: Milo Kim <milo.kim@ti.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>