V4L/DVB: drivers/video/omap2/displays: add missing mutex_unlock

Add a mutex_unlock missing on the error paths.  The use of the mutex is
balanced elsewhere in the file.

The semantic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@
expression E1;
@@

* mutex_lock(E1,...);
  <+... when != E1
  if (...) {
    ... when != E1
*   return ...;
  }
  ...+>
* mutex_unlock(E1,...);
// </smpl>
Signed-off-by: Julia Lawall <julia@diku.dk>
Acked-by: Mike Isely <isely@pobox.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/video/omap2/displays/panel-acx565akm.c

@@ -592,7 +592,7 @@ static int acx_panel_power_on(struct omap_dss_device *dssdev)
 	r = omapdss_sdi_display_enable(dssdev);
 	if (r) {
 		pr_err("%s sdi enable failed\n", __func__);
-		return r;
+		goto fail_unlock;
 	}
 
 	/*FIXME tweak me */
@@ -633,6 +633,8 @@ static int acx_panel_power_on(struct omap_dss_device *dssdev)
 	return acx565akm_bl_update_status(md->bl_dev);
 fail:
 	omapdss_sdi_display_disable(dssdev);
+fail_unlock:
+	mutex_unlock(&md->mutex);
 	return r;
 }