Unfortunately, it appears our fix in:
commit b5d29843 ("drm/atomic_helper: Allow DPMS On<->Off changes
for unregistered connectors")

Which attempted to work around the problems introduced by:
commit 4d802739 ("drm/atomic_helper: Disallow new modesets on
unregistered connectors")

Is still not the right solution, as modesets can still be triggered
outside of drm_atomic_set_crtc_for_connector().

So in order to fix this, while still being careful that we don't break
modesets that a driver may perform before being registered with
userspace, we replace connector->registered with a tristate member,
connector->registration_state. This allows us to keep track of whether
or not a connector is still initializing and hasn't been exposed to
userspace, is currently registered and exposed to userspace, or has been
legitimately removed from the system after having once been present.

Using this info, we can prevent userspace from performing new modesets
on unregistered connectors while still allowing the driver to perform
modesets on unregistered connectors before the driver has finished being
registered.

Changes since v1:
- Fix WARN_ON() in drm_connector_cleanup() that CI caught with this
  patchset in igt@drv_module_reload@basic-reload-inject and
  igt@drv_module_reload@basic-reload by checking if the connector is
  registered instead of unregistered, as calling drm_connector_cleanup()
  on a connector that hasn't been registered with userspace yet should
  stay valid.
- Remove unregistered_connector_check(), and just go back to what we
  were doing before in commit 4d802739 ("drm/atomic_helper: Disallow
  new modesets on unregistered connectors") except replacing
  READ_ONCE(connector->registered) with drm_connector_is_unregistered().
  This gets rid of the behavior of allowing DPMS On<->Off, but that should
  be fine as it's more consistent with the UAPI we had before - danvet
- s/drm_connector_unregistered/drm_connector_is_unregistered/ - danvet
- Update documentation, fix some typos.

Fixes: b5d29843 ("drm/atomic_helper: Allow DPMS On<->Off changes for unregistered connectors")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: stable@vger.kernel.org
Cc: David Airlie <airlied@linux.ie>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181016203946.9601-1-lyude@redhat.com