drm: renesas: shmobile: Use suspend/resume helpers

Replace the custom suspend/resume handling by calls into
drm_mode_config_helper_{suspend,resume}().

Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/a52147ca6033e0a59675f37f0326c7404fc5919c.1694767209.git.geert+renesas@glider.be