lib/igt_kms, tests/testdisplay: allow probing of new connector modes

Fixup some fallout from the connector probing changes so testdisplay -m
will pick up newly hotplugged displays correctly.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org.

tests/testdisplay.h

@@ -32,4 +32,4 @@ gboolean testdisplay_setup_hotplug(void);
 void testdisplay_cleanup_hotplug(void);
 
 /* called by the hotplug code */
-int update_display(void);
+int update_display(bool probe);