drm/omap: venc: Use drm_display_mode natively

Replace internal usage of struct videomode with struct drm_display_mode
in order to avoid converting needlessly between the data structures.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>