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 <[e-mail]>
Fixes: e21dd290881b ("drm/panfrost: Enable devfreq to work without regulator")
Signed-off-by: Steven Price <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/panfrost/panfrost_devfreq.c

···
 	if (err) {
 		dev_err(dev, "Cannot set frequency %lu (%d)\n", target_rate,
 			err);
-		regulator_set_voltage(pfdev->regulator, pfdev->devfreq.cur_volt,
-				      pfdev->devfreq.cur_volt);
+		if (pfdev->regulator)
+			regulator_set_voltage(pfdev->regulator,
+					      pfdev->devfreq.cur_volt,
+					      pfdev->devfreq.cur_volt);
 		return err;
 	}