OMAPDSS: add OMAP_DSS_CHANNEL_WB to 'enum omap_channel'

'enum omap_channel' is missing the channel for writeback. Add that.
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@ti.com>

include/video/omapdss.h

@@ -87,6 +87,7 @@ enum omap_channel {
 	OMAP_DSS_CHANNEL_DIGIT	= 1,
 	OMAP_DSS_CHANNEL_LCD2	= 2,
 	OMAP_DSS_CHANNEL_LCD3	= 3,
+	OMAP_DSS_CHANNEL_WB	= 4,
 };
 
 enum omap_color_mode {