OMAP: DSS2: OMAPFB: fix crash when panel driver was not loaded

If the panel's probe had failed, omapfb would still go on, eventually
crashing.

A better fix would be to handle each display properly, and leaving just
the failed display out. But that is a bigger change.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>

drivers/video/omap2/omapfb/omapfb-main.c

@@ -2115,6 +2115,11 @@ static int omapfb_probe(struct platform_device *pdev)
 	dssdev = NULL;
 	for_each_dss_dev(dssdev) {
 		omap_dss_get_device(dssdev);
+		if (!dssdev->driver) {
+			dev_err(&pdev->dev, "no driver for display\n");
+			r = -EINVAL;
+			goto cleanup;
+		}
 		fbdev->displays[fbdev->num_displays++] = dssdev;
 	}