drm/rcar-du: Convert drm_atomic_helper_suspend/resume()

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

remove suspend_state field from the rcar_du_device
structure as it is no more required.

With this conversion, also drm_fbdev_cma_set_suspend_unlocked()
will left with no consumer. So this function can be removed.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180918163903.GA11172@jordon-HP-15-Notebook-PC