backlight: backlight: Introduce backlight_get_brightness()

Based on an idea from Emil Velikov, add a helper that checks
backlight_is_blank() and return 0 as brightness if display is blank or
the property value if not.

This allows us to simplify the update_status() implementation
in most of the backlight drivers.

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>