drm/bridge: Add a drm_bridge_state object

One of the last remaining objects to not have its atomic state.

This is being motivated by our attempt to support runtime bus-format
negotiation between elements of the bridge chain.
This patch just paves the road for such a feature by adding a new
drm_bridge_state object inheriting from drm_private_obj so we can
re-use some of the existing state initialization/tracking logic.

v10:
* Add changelog to the commit message

v9:
* Clarify the fact that the bridge->atomic_reset() and
  {connector,plane,crtc,...}->reset() semantics are different
* Move the drm_atomic_private_obj_init() call back to
  drm_bridge_attach()
* Check the presence of ->atomic_duplicate_state instead of
  ->atomic_reset in drm_atomic_add_encoder_bridges()
* Fix copy&paste errors in the atomic bridge state helpers doc
* Add A-b/R-b tags

v8:
* Move bridge state helpers out of the CONFIG_DEBUGFS section

v7:
* Move helpers, struct-defs, ... to atomic helper files to avoid the
  drm -> drm_kms_he...