drm/panfrost: Add missing check for pfdev->regulator

When modifying panfrost_devfreq_target() to support a device without a
regulator defined I missed the check on the error path. Let's add it.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: e21dd290 ("drm/panfrost: Enable devfreq to work without regulator")
Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190822093218.26014-1-steven.price@arm.com

drivers/gpu/drm/panfrost/panfrost_devfreq.c

@@ -53,7 +53,9 @@ static int panfrost_devfreq_target(struct device *dev, unsigned long *freq,
 	if (err) {
 		dev_err(dev, "Cannot set frequency %lu (%d)\n", target_rate,
 			err);
-		regulator_set_voltage(pfdev->regulator, pfdev->devfreq.cur_volt,
+		if (pfdev->regulator)
+			regulator_set_voltage(pfdev->regulator,
+					      pfdev->devfreq.cur_volt,
 					      pfdev->devfreq.cur_volt);
 		return err;
 	}