backlight: pwm_bl: Switch to using "atomic" PWM API

The "atomic" API allows us to configure PWM period and duty_cycle and
enable it in one call.

The patch also moves the pwm_init_state just before any use of the
pwm_state struct, this fixes a potential bug where pwm_get_state
can be called before pwm_init_state.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>