OMAPDSS: Use WB fifo for GFX overlay

OMAP4's GFX overlay has smaller fifo than the rest of the overlays
(including writeback "overlay"). This seems to be the reason for
underflows in some more demanding scenarios.

We can avoid the problems by using the WB fifo for GFX overlay, and vice
versa. WB usage is not supported yet, but when it will, it should
perform just fine with smaller fifo as there are no hard realtime
constraints with WB.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
diff --git a/drivers/video/omap2/dss/dispc.h b/drivers/video/omap2/dss/dispc.h
index 92d8a9b..42e56cc 100644
--- a/drivers/video/omap2/dss/dispc.h
+++ b/drivers/video/omap2/dss/dispc.h
@@ -36,6 +36,7 @@
 #define DISPC_CONTROL2			0x0238
 #define DISPC_CONFIG2			0x0620
 #define DISPC_DIVISOR			0x0804
+#define DISPC_GLOBAL_BUFFER		0x0800
 #define DISPC_CONTROL3                  0x0848
 #define DISPC_CONFIG3                   0x084C
 
@@ -355,6 +356,8 @@
 		return 0x014C;
 	case OMAP_DSS_VIDEO3:
 		return 0x0300;
+	case OMAP_DSS_WB:
+		return 0x0500;
 	default:
 		BUG();
 		return 0;
@@ -517,6 +520,7 @@
 	case OMAP_DSS_VIDEO2:
 		return 0x0018;
 	case OMAP_DSS_VIDEO3:
+	case OMAP_DSS_WB:
 		return 0x0088;
 	default:
 		BUG();