drm/nouveau/debugfs: fix runtime pm imbalance on error

pm_runtime_get_sync() increments the runtime PM usage counter even
the call returns an error code. Thus a pairing decrement is needed
on the error handling path to keep the counter balanced.
Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_debugfs.c

@@ -181,8 +181,11 @@ nouveau_debugfs_pstate_set(struct file *file, const char __user *ubuf,
 	}
 
 	ret = pm_runtime_get_sync(drm->dev);
-	if (ret < 0 && ret != -EACCES)
+	if (ret < 0 && ret != -EACCES) {
+		pm_runtime_put_autosuspend(drm->dev);
 		return ret;
+	}
+
 	ret = nvif_mthd(ctrl, NVIF_CONTROL_PSTATE_USER, &args, sizeof(args));
 	pm_runtime_put_autosuspend(drm->dev);
 	if (ret < 0)