drm/fsl-dcu: Use drm_mode_config_helper_suspend/resume()

Replace driver's code with the generic helpers that do the same thing.

Cc: Stefan Agner <stefan@agner.ch>
Cc: Alison Wang <alison.wang@freescale.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171114212517.23536-5-noralf@tronnes.org