drm/omap: venc: Simplify mode setting by caching configuration

The mode setting handler of the VENC stores the video mode internally,
to then convert it to a configuration when programming the hardware. The
stored mode is otherwise unused. Cache the configuration directly
instead.

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>