video: of: Constify node argument to display timing functions

The node pointer passed to the display timing functions is never
modified, make it const.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>