drm/omap: drop unused DSS next pointer

Since all encoders and panels are using the bridge API now,
we next pointer is no longer useful and can be dropped.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-45-tomi.valkeinen@ti.com