OMAP: DSS2: OMAPFB: Convert the memory region locking to rwsem

R/W semaphore is a good fit for the memory region locking pattern.
So use it.

Signed-off-by: Ville Syrjälä <ville.syrjala@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 69bcbfc..3a10146 100644
--- a/drivers/video/omap2/omapfb/omapfb-ioctl.c
+++ b/drivers/video/omap2/omapfb/omapfb-ioctl.c
@@ -198,13 +198,7 @@
 
 	rg = ofbi->region;
 
-	/* FIXME probably should be a rwsem ... */
-	mutex_lock(&rg->mtx);
-	while (rg->ref) {
-		mutex_unlock(&rg->mtx);
-		schedule();
-		mutex_lock(&rg->mtx);
-	}
+	down_write(&rg->lock);
 
 	if (atomic_read(&rg->map_count)) {
 		r = -EBUSY;
@@ -235,7 +229,7 @@
 	}
 
  out:
-	mutex_unlock(&rg->mtx);
+	up_write(&rg->lock);
 
 	return r;
 }