video: backlight: lp855x: Get PWM for PWM mode during probe

Also deprecate the pwm-period DT property, as it is now redundant
(pwms property already contains period value).

Signed-off-by: Artur Weber <aweber.kernel@gmail.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230519180728.2281-3-aweber.kernel@gmail.com