OMAP: DSS2: Fix crash when panel doesn't define enable_te()

DSI driver didn't check if the panel driver actually implements
enable_te().
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@nokia.com>

drivers/video/omap2/dss/dsi.c

@@ -2932,11 +2932,15 @@ static int dsi_set_update_mode(struct omap_dss_device *dssdev,
 
 static int dsi_set_te(struct omap_dss_device *dssdev, bool enable)
 {
-	int r;
+	int r = 0;
+
+	if (dssdev->driver->enable_te) {
 		r = dssdev->driver->enable_te(dssdev, enable);
 		/* XXX for some reason, DSI TE breaks if we don't wait here.
 		 * Panel bug? Needs more studying */
 		msleep(100);
+	}
+
 	return r;
 }