UPSTREAM: backlight: pwm_bl: Use pwm_get_args() where appropriate

UPSTREAM: backlight: pwm_bl: Use pwm_get_args() where appropriate

The PWM framework has clarified the concept of reference PWM config (the
platform dependent config retrieved from the DT or the PWM lookup table)
and real PWM state.

Use pwm_get_args() when the PWM user wants to retrieve this reference
config and not the current state.

This is part of the rework allowing the PWM framework to support
hardware readout and expose real PWM state even when the PWM has just
been requested (before the user calls pwm_config/enable/disable()).

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
(cherry picked from commit 6cb9644db7364ff5d2980ccd365b8cb684145327)

Change-Id: Idc29a17716d439962b50db156c13b65abd852d90
Signed-off-by: David Wu <david.wu@rock-chips.com>