drm/drm_stub: Remove unnecessary null check before kfree.

kfree on a null argument is a no-op.
Silences the following smatch warning:
drivers/gpu/drm/drm_stub.c:496 drm_put_dev() info:
redundant null check on dev->devname calling kfree()
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_stub.c

@@ -492,10 +492,7 @@ void drm_put_dev(struct drm_device *dev)
 	drm_put_minor(&dev->primary);
 
 	list_del(&dev->driver_item);
-	if (dev->devname) {
-		kfree(dev->devname);
-		dev->devname = NULL;
-	}
+	kfree(dev->devname);
 	kfree(dev);
 }
 EXPORT_SYMBOL(drm_put_dev);