backlight: Show brightness even if get_brightness is not implemented

Many implementations of get_brightness operation returns only value
of props.brightness field. This change makes such implementations
unnecessary.

Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>