Fix accessibility not getting initialized in gtk_init() if a default display is alrea...

Fix accessibility not getting initialized in gtk_init() if a default display is already set

_gtk_accessibility_init() only gets called if the default
display changes, but in case gdk_init() is called before gtk_init()
the default display is already set and no property notification occurs.

This can happen quite easily in pygobject where
"from gi.repository import Gdk, Gtk"
will call gdk_init() followed by gtk_init() in the Python overrides.

This fixes it by checking for a default display in all cases.