drm/omap: dsi: Fixup video mode in .set_config() operation

The DSI encoder modifies the passed videomode to take the requirements
of the internal DISPC-DSI bus into account in the .enable_video_output()
operation. This should be performed in the .check_timings() operation
instead. There is however no .check_timings() operation as the DSI
encoder uses a custom API, so move it to the closest match which is the
.set_config() 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>