drm/omap: Use atomic suspend/resume helpers

Instead of rolling out custom suspend/resume implementations based on
state information stored in the driver's data structures, use the atomic
suspend/resume helpers that rely on a DRM atomic state object.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>