ARM: s3c24xx: Switch to atomic pwm API in rx1950

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>