backlight: backlight: Drop backlight_put()

There are no external users of backlight_put(). Drop it and open code
the two users in backlight.c.

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>