drm/omap: Make the video_mode pointer to .set_timings() const

The .set_timings() operations of the omap_dss_device instances don't
need to modify the passed timings. Make the pointer const.

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>