pwm: mtk-disp: Implement atomic API .get_state()

Switch the driver to support the .get_state() method.

Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
[thierry.reding@gmail.com: add missing linux/bitfield.h include]
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>