drm/hdcp: update content protection property with uevent

drm function is defined and exported to update a connector's
content protection property state and to generate a uevent along
with it.

Pekka have completed the Weston DRM-backend review in
https://gitlab.freedesktop.org/wayland/weston/merge_requests/48


and the UAPI for HDCP 2.2 looks good.

The userspace is accepted in Weston.

v2:
  Update only when state is different from old one.
v3:
  KDoc is added [Daniel]
v4:
  KDoc is extended bit more [pekka]
v5:
  Uevent usage is documented at kdoc of "Content Protection" also
  [pekka]

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/320963/?series=57232&rev=14