backlight: gpio: Pull gpio_backlight_initial_power_state() into probe

The probe function in the gpio-backlight driver is quite short. If we
pull gpio_backlight_initial_power_state() into probe we can drop two
more fields from struct gpio_backlight and shrink the driver code.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>