video: pwm-backlight: Honour discrete brightness levels from dt
If probed from device tree we have an array of pwm duty cycle levels
in the brightness-levels property. Although the driver parsed the property,
the values have been ignored. Make use of these flags so that the driver
can work for example with inverted PWMs.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>