backlight: atmel-pwm-bl: refactor gpio_on handling

Add helper function to control the gpio_on signal.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>