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

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

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.

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

The same fix was applied to gtk3 in !26 where calling gdk_init()
before gtk_init() would trigger this case. With gdk_init() gone
in master this is less likely to happen, but still possible
if gdk_display_open() is called before gtk_init().

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