drm: Do not force 1024x768 modes on unknown connectors

Only fallback to a set of default modes on a connector iff that
connector is known to be connected. The issue occurs that with limited
hardware which cannot probe a connector and so reports the
connector status as unknown will then attempt to retrieve the modes for
it during drm_helper_probe_single_connector_modes(). Should that fail,
the helper then generates a default set which fools the fb_helper and
causes havoc with the console and beyond.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_crtc_helper.c

@@ -115,11 +115,10 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector,
 	}
 
 	count = (*connector_funcs->get_modes)(connector);
-	if (!count) {
+	if (count == 0 && connector->status == connector_status_connected)
 		count = drm_add_modes_noedid(connector, 1024, 768);
-		if (!count)
-			return 0;
-	}
+	if (count == 0)
+		goto prune;
 
 	drm_mode_connector_list_update(connector);