Stop using the legacy PWM API which only still exists because there are
some users left.

Note this change make use of the fact that the value of struct
pwm_state::duty_cycle doesn't matter for a disabled PWM and so its value
can stay constant simplifying the code a bit.

A side effect of the conversion is that the pwm isn't stopped in
rx1950_backlight_init() by the call to pwm_apply_args() just before
reenabling it when rx1950_lcd_power(1) is called.
Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>