OMAPDSS: DISPC: pclk & lclk rates for writeback

Change the dispc_plane_pclk_rate and dispc_plane_lclk_rate functions to
return 0 if the given plane is the writeback plane. The clocks are not
valid for WB, but returning 0 from these functions instead of running
into BUG() will simplify the code that uses these functions.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

drivers/video/omap2/dss/dispc.c

@@ -3232,14 +3232,24 @@ unsigned long dispc_core_clk_rate(void)
 
 static unsigned long dispc_plane_pclk_rate(enum omap_plane plane)
 {
-	enum omap_channel channel = dispc_ovl_get_channel_out(plane);
+	enum omap_channel channel;
+
+	if (plane == OMAP_DSS_WB)
+		return 0;
+
+	channel = dispc_ovl_get_channel_out(plane);
 
 	return dispc_mgr_pclk_rate(channel);
 }
 
 static unsigned long dispc_plane_lclk_rate(enum omap_plane plane)
 {
-	enum omap_channel channel = dispc_ovl_get_channel_out(plane);
+	enum omap_channel channel;
+
+	if (plane == OMAP_DSS_WB)
+		return 0;
+
+	channel	= dispc_ovl_get_channel_out(plane);
 
 	return dispc_mgr_lclk_rate(channel);
 }