OMAPDSS: DISPC: Add manager like functions for writeback

Add functions to enable writeback, and set/check state of GO bit. These bits are
identical in behaviour with the corresponding overlay manager bits. Configure
them in a similar way to mgr_enable() and mgr_go_* functions. Add a helper to
get the FRAMEDONE irq corresponding to writeback.

Signed-off-by: Archit Taneja <archit@ti.com>
diff --git a/drivers/video/omap2/dss/dss.h b/drivers/video/omap2/dss/dss.h
index 8677f51..6728892 100644
--- a/drivers/video/omap2/dss/dss.h
+++ b/drivers/video/omap2/dss/dss.h
@@ -486,6 +486,11 @@
 void dispc_mgr_setup(enum omap_channel channel,
 		struct omap_overlay_manager_info *info);
 
+u32 dispc_wb_get_framedone_irq(void);
+bool dispc_wb_go_busy(void);
+void dispc_wb_go(void);
+void dispc_wb_enable(bool enable);
+bool dispc_wb_is_enabled(void);
 void dispc_wb_set_channel_in(enum dss_writeback_channel channel);
 int dispc_wb_setup(const struct omap_dss_writeback_info *wi,
 		bool mem_to_mem, const struct omap_video_timings *timings);