drm/omap: Don't store video mode internally for external encoders

The omap_dss_device .set_timings() operation for external encoders
stores the video mode in the device data structure. That mode is then
never used again. Drop it.

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>