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>