xmesa: call _glapi_set_dispatch() for all xserver DDXes.

This is to unify the xmesa code across xserver DDX'es. The call is intented for
XGL, but it does not hurt to call for other DDX'es. In fact it was not guarded
against XGL when it was first added in xserver.

src/mesa/drivers/x11/xm_api.c

@@ -1869,9 +1869,7 @@ PUBLIC
 GLboolean XMesaForceCurrent(XMesaContext c)
 {
    if (c) {
-#ifdef XGLServer
       _glapi_set_dispatch(c->mesa.CurrentDispatch);
-#endif
 
       if (&(c->mesa) != _mesa_get_current_context()) {
 	 _mesa_make_current(&c->mesa, c->mesa.DrawBuffer, c->mesa.ReadBuffer);