drm/omap: simplify omap_display_id

We no longer need to check for the DSS API, since all encoders,
panels and connectors have been converted to the bridge API.

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/20201215104657.802264-44-tomi.valkeinen@ti.com