drm/omap: dss: Move src and dst check and set to connection handlers

The encoders duplicate the same omap_dss_device src and dst fields set
and checks in their connect and disconnect handlers. Move the code to
the connect and disconnect wrappers.

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>