gtk_init: Fix debug flags handling when a display is already open

gtk_init: Fix debug flags handling when a display is already open

In PyGObject gdk_init() is called before gtk_init() and thus there is
already a default display open when gtk_init() is called.
The code assigning the display to the debug_flags struct gets only
called when the default display changes, which never happens
when there already is one. As a result GTK_DEBUG=interactive
doesn't do anyting with Python apps.

This makes it call the change callback in case a display is already
there.

See https://gitlab.gnome.org/GNOME/pygobject/issues/166