drm/property: Enforce more lifetime rules

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 e0f32f78e5 (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

drivers/gpu/drm/drm_mode_object.c

@@ -224,12 +224,26 @@ EXPORT_SYMBOL(drm_mode_object_get);
  * This attaches the given property to the modeset object with the given initial
  * value. Currently this function cannot fail since the properties are stored in
  * a statically sized array.
+ *
+ * Note that all properties must be attached before the object itself is
+ * registered and accessible from userspace.
  */
 void drm_object_attach_property(struct drm_mode_object *obj,
 				struct drm_property *property,
 				uint64_t init_val)
 {
 	int count = obj->properties->count;
+	struct drm_device *dev = property->dev;
+
+
+	if (obj->type == DRM_MODE_OBJECT_CONNECTOR) {
+		struct drm_connector *connector = obj_to_connector(obj);
+
+		WARN_ON(!dev->driver->load &&
+			connector->registration_state == DRM_CONNECTOR_REGISTERED);
+	} else {
+		WARN_ON(!dev->driver->load && dev->registered);
+	}
 
 	if (count == DRM_OBJECT_MAX_PROPERTY) {
 		WARN(1, "Failed to attach object property (type: 0x%x). Please "