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

@@ -697,12 +697,16 @@ static int dss_get_clocks(void)
 
 	dss.dss_clk = clk;
 
+	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;