video: ssd1307fb: Convert to atomic PWM API

Use the atomic API wherever appropriate and get rid of pwm_apply_args()
call (the reference period and polarity are now explicitly set when
calling pwm_apply_state()).

We also make use of the pwm_set_relative_duty_cycle() helper to ease
relative to absolute duty_cycle conversion.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200324170532.44384-4-andriy.shevchenko@linux.intel.com