OMAPDSS: pll: NULL dereference in error handling

The regulator_disable() doesn't accept NULL pointers.

Signed-off-by: Dan Carpenter <redacted>

diff --git a/drivers/video/fbdev/omap2/dss/pll.c b/drivers/video/fbdev/omap2/dss/pll.c
index 50bc62c5..335ffac 100644
--- a/drivers/video/fbdev/omap2/dss/pll.c
+++ b/drivers/video/fbdev/omap2/dss/pll.c

@@ -97,7 +97,8 @@ int dss_pll_enable(struct dss_pll *pll)
 	return 0;
 
 err_enable:
-	regulator_disable(pll->regulator);
+	if (pll->regulator)
+		regulator_disable(pll->regulator);
 err_reg:
 	clk_disable_unprepare(pll->clkin);
 	return r;