backlight: lm3630a: Turn off both led strings when display is blank

Use backlight_is_blank() to determine if the led strings should be turned
off in the update_status() functions of both strings.

Signed-off-by: Maximilian Weigand <mweigand@mweigand.net>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://lore.kernel.org/r/20230510085239.1253896-1-mweigand2017@gmail.com


Signed-off-by: Lee Jones <lee@kernel.org>