pwm_backlight: Validate dft_brightness in main probe function

Move the dft_brightness validity check from the DT parsing function to the
main probe. In this way we can validate it in case we are booting with or
without DT.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>