vnc-display: Correct arguments to a signal handler

Correct arguments to handler of Vnc.Display.vnc_auth_credential signal.

https://bugzilla.gnome.org/show_bug.cgi?id=736099
vnc-display: Correctly cast to DisplayCredential

DisplayCredential is not a class so you can't use 'as' operator to cast
to this type.

This together with previous patch, fixes a crash that always happens if
you try to connect to a VNC box that has password on it.

https://bugzilla.gnome.org/show_bug.cgi?id=736099