backlight: cr_bllcd: Introduce gpio-backlight semantics

cr_bllcd can turn backlight ON or OFF.

Fix semantitics so they equals what we know from gpio-backlight.

 brightness == 0   => backlight off
 brightness == 1   => backlight on

Use the backlight_get_brightness() helper to simplify the code.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>