Set private port's user data on creation

Set private port's user data on creation

With separate port creation and user data setting calls, the following
scenario is problematic:

    Source adds a port (private data is not set yet)
      Graph user's "port added" callback is called
        This callback connects the new port to a sink port
          This sink's "port connected" method is called
            This sink creates a notification iterator on the connected
            port's connection
              The source's notification iterator initialization method
              is called with the added port with NO private data yet

With this change, it is guaranteed that as soon as the port can be
accessed by any user, its private data is set.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>