drm/omap: panel-dsi-cm: Store source pointer internally

The source pointer will be removed to the omap_dss_device structure.
Store it internally in the DSI panel driver data.

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>