drm: rcar-du: dsi: Properly stop video mode TX

The driver does not explicitly stop the video mode transmission when
disabling the output. While this doesn't seem to be causing any issues,
lets follow the steps described in the documentation and add a
rcar_mipi_dsi_stop_video() which stop the video mode transmission. This
function will also be used in later patches to stop the video
transmission even if the DSI IP is not shut down.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>