OMAPDSS: do not fail if dpll4_m4_ck is missing

Do not fail if dpll4_m4_ck is missing. The clock is not there on omap24xx,
so this should not be a hard error.

The patch retains the functionality before the commit 185bae10 (OMAPDSS:
DSS: Cleanup cpu_is_xxxx checks).
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

drivers/video/omap2/dss/dss.c

@@ -697,11 +697,15 @@ static int dss_get_clocks(void)
 
 	dss.dss_clk = clk;
 
-	clk = clk_get(NULL, dss.feat->clk_name);
-	if (IS_ERR(clk)) {
-		DSSERR("Failed to get %s\n", dss.feat->clk_name);
-		r = PTR_ERR(clk);
-		goto err;
+	if (dss.feat->clk_name) {
+		clk = clk_get(NULL, dss.feat->clk_name);
+		if (IS_ERR(clk)) {
+			DSSERR("Failed to get %s\n", dss.feat->clk_name);
+			r = PTR_ERR(clk);
+			goto err;
+		}
+	} else {
+		clk = NULL;
 	}
 
 	dss.dpll4_m4_ck = clk;