OMAP: DSS2: move set/get_mirror()

Move set/get_mirror() from omap_dss_device to omap_dss_driver.

This is part of a larger patch-set, which moves the control from omapdss
driver to the display driver.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>
diff --git a/drivers/video/omap2/displays/panel-taal.c b/drivers/video/omap2/displays/panel-taal.c
index 5f78d38..4d64449 100644
--- a/drivers/video/omap2/displays/panel-taal.c
+++ b/drivers/video/omap2/displays/panel-taal.c
@@ -797,16 +797,20 @@
 
 	dev_dbg(&dssdev->dev, "mirror %d\n", enable);
 
+	dsi_bus_lock();
 	if (td->enabled) {
 		r = taal_set_addr_mode(td->rotate, enable);
-
 		if (r)
-			return r;
+			goto err;
 	}
 
 	td->mirror = enable;
 
+	dsi_bus_unlock();
 	return 0;
+err:
+	dsi_bus_unlock();
+	return r;
 }
 
 static bool taal_get_mirror(struct omap_dss_device *dssdev)