drm/mediatek: Convert drm_atomic_helper_suspend/resume()

convert drm_atomic_helper_suspend/resume() to use
drm_mode_config_helper_suspend/resume().

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Ajit Negi <ajitn.linux@gmail.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>