backlight: gpio_backlight: Simplify update_status()

Introduce the use of backlight_get_brightness() to simplify the
update_status() operation. With the simpler implementation drop the
gpio_backlight_get_next_brightness() helper as it was now a
one-liner.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>