drm/omap: dss: Move debug message and checks to connection handlers

The connectors, encoders and display duplicate the same debug messages
and connection checks in their omap_dss_device connect and disconnect
handlers. Move the code to the connect and disconnect wrappers.

To simplify the code the connect function returns -EBUSY unconditionally
if the device is already connected. This doesn't cause any change in
practice: the connect handler of displays is never called on a connected
device as it is only invoked during omapdrm initialization.

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>