drm: uevent for connector status change

DRM API for generating uevent for a status changes of connector's
property.

This uevent will have following details related to the status change:

  HOTPLUG=1, CONNECTOR=<connector_id> and PROPERTY=<property_id>

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:
  Minor fixes at KDoc comments [Daniel]
v3:
  Check the property is really attached with connector [Daniel]
v4:
  Typos and string length suggestions are addressed [Sean]

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sean Paul <sean@poorly.run>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/320961/?series=57232&rev=14