drm/nouveau/debugfs: fix file release memory leak

When using single_open() for opening, single_release() should be
called, otherwise the 'op' allocated in single_open() will be leaked.

Fixes: 6e9fc177 ("drm/nouveau/debugfs: add copy of sysfs pstate interface ported to debugfs")
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Reviewed-by: NKarol Herbst <kherbst@redhat.com>
Signed-off-by: NKarol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210911075023.3969054-2-yangyingliang@huawei.comSigned-off-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/nouveau/nouveau_debugfs.c

@@ -207,6 +207,7 @@ static const struct file_operations nouveau_pstate_fops = {
 	.open = nouveau_debugfs_pstate_open,
 	.read = seq_read,
 	.write = nouveau_debugfs_pstate_set,
+	.release = single_release,
 };
 
 static struct drm_info_list nouveau_debugfs_list[] = {