backlight: jornada720_bl: Introduce backlight_is_blank()

Use the backlight_is_blank() helper to simplify the code a bit.

The jornada720_bl driver distinguish between backlight off
and brightness set to 0.

Thus this driver turn off backlight only when backlight_is_blank()
returns true.

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>