Add support for privacy-screen consumers to register a notifier to
be notified of external (e.g. done by the hw itself on a hotkey press)
state changes.

Changes in v2:
- Drop WARN_ON(mutex_is_locked(&priv->lock)) check in
  drm_privacy_screen_call_notifier_chain() it may be locked by
  another thread, which would lead to a false-positive triggering
  of the check
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211005202322.700909-5-hdegoede@redhat.com