drm/omap: displays: Remove input omap_dss_device from panel data

All connectors, encoders and panels store a pointer to their input
omap_dss_device in the panel driver data structure. This duplicates the
src field in the omap_dss_device structure. Remove the private copy and
use the src field.

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