Properties can't be attached after registering, userspace would get
confused (no one bothers to reprobe really).

- Add kerneldoc
- Enforce this with some checks. This needs a somewhat ugly check
  since connectors can be added later on, but we still need to attach
  all properties before they go public.

Note that we already enforce that properties themselves are created
before the entire device is registered.

Unfortunately this doesn't work for drivers which have a ->load
callback, see

commit e0f32f78 (tag:
drm-misc-next-fixes-2019-09-18)
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue Sep 17 14:09:35 2019 +0200

    drm/kms: Duct-tape for mode object lifetime checks

for the full story.

v2: Fix the superflous newline (Jani) and add commit citation to
explain why we need to check for dev->registered (Thierry).

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Rajat Jain <rajatja@google.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191023144953.28190-1-daniel.vetter@ffwll.ch