drm/omap: displays: Remove videomode from omap_dss_device structure

The omap_dss_device structure stores a videomode. All the connector and
panel drivers that use omap_dss_device also store the videomode in their
own panel_drv_data structures. There's no need to duplicate, remove the
videomode field from omap_dss_device.

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>