drm/omap: Remove omap_dss_device dst field

The field is only used in a safety check during device
connection/disconnection, where the src field can be easily used
instead. Remove it and use src.

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>