OMAPDSS: pll: NULL dereference in error handling

The regulator_disable() doesn't accept NULL pointers.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

drivers/video/fbdev/omap2/dss/pll.c

@@ -97,6 +97,7 @@ int dss_pll_enable(struct dss_pll *pll)
 	return 0;
 
 err_enable:
+	if (pll->regulator)
 		regulator_disable(pll->regulator);
 err_reg:
 	clk_disable_unprepare(pll->clkin);