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>
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