OMAP: DSS2: check lock_fb_info() return value

Give up if lock_fb_info() fails, following the same convention as other
lock_fb_info() users.

Signed-off-by: Jani Nikula <ext-jani.1.nikula@nokia.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>
diff --git a/drivers/video/omap2/omapfb/omapfb-ioctl.c b/drivers/video/omap2/omapfb/omapfb-ioctl.c
index 2c0f01c..9c73618 100644
--- a/drivers/video/omap2/omapfb/omapfb-ioctl.c
+++ b/drivers/video/omap2/omapfb/omapfb-ioctl.c
@@ -183,7 +183,8 @@
 	struct omapfb2_device *fbdev = ofbi->fbdev;
 	int r;
 
-	lock_fb_info(fbi);
+	if (!lock_fb_info(fbi))
+		return -ENODEV;
 	omapfb_lock(fbdev);
 
 	r = omapfb_update_window_nolock(fbi, x, y, w, h);