backlight: pwm_bl: Configure pwm only once per backlight toggle

When the function pwm_backlight_update_status() was called with
brightness > 0, pwm_get_state() was called twice (once directly and once
in compute_duty_cycle). Also pwm_apply_state() was called twice (once in
pwm_backlight_power_on() and once directly).

Optimize this to do both calls only once.

Note that with this affects the order of regulator and PWM setup. It's
not expected to have a relevant effect on hardware. The rationale for
this is that the regulator (and the GPIO) are reasonable to switch in
pwm_backlight_power_on()/pwm_backlight_power_off() but the PWM has
nothing to do with power. (The post_pwm_on_delay and pwm_off_delay are
still there though.)

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230120120018.161103-2-u.kleine-koenig@pengutronix.de