OMAPDSS: cleanup WB enable/is_enabled functions

Instead of doing direct register reads/writes, dispc_wb_enable() and
dispc_wb_is_enabled() functions can use the common overlay functions to
set and check the enable bit.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

drivers/video/omap2/dss/dispc.c

@@ -2810,12 +2810,12 @@ void dispc_mgr_disable_sync(enum omap_channel channel)
 
 void dispc_wb_enable(bool enable)
 {
-	REG_FLD_MOD(DISPC_OVL_ATTRIBUTES(OMAP_DSS_WB), enable ? 1 : 0, 0, 0);
+	dispc_ovl_enable(OMAP_DSS_WB, enable);
 }
 
 bool dispc_wb_is_enabled(void)
 {
-	return REG_GET(DISPC_OVL_ATTRIBUTES(OMAP_DSS_WB), 0, 0);
+	return dispc_ovl_enabled(OMAP_DSS_WB);
 }
 
 static void dispc_lcd_enable_signal_polarity(bool act_high)