OMAPDSS: add OMAP_DSS_CHANNEL_WB to 'enum omap_channel'

'enum omap_channel' is missing the channel for writeback. Add that.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
diff --git a/include/video/omapdss.h b/include/video/omapdss.h
index f001a35..0fedd93 100644
--- a/include/video/omapdss.h
+++ b/include/video/omapdss.h
@@ -87,6 +87,7 @@
 	OMAP_DSS_CHANNEL_DIGIT	= 1,
 	OMAP_DSS_CHANNEL_LCD2	= 2,
 	OMAP_DSS_CHANNEL_LCD3	= 3,
+	OMAP_DSS_CHANNEL_WB	= 4,
 };
 
 enum omap_color_mode {