drm/omap: drop dssdev display field

All displays are now using a drm_panel or a drm_bridge that models the
connector instead of dssdev, so this field is always 0 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/20201215104657.802264-47-tomi.valkeinen@ti.com