drm/omap: hdmi: Constify video mode and related pointers

Constify many pointers to struct videomode, as well as pointers to
container structures, to ensure the video mode isn't modified after
the .check_timings() operation.

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>