drm/nouveau/debugfs: fix file release memory leak

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: 6e9fc177399f ("drm/nouveau/debugfs: add copy of sysfs pstate interface ported to debugfs")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210911075023.3969054-2-yangyingliang@huawei.com
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>