drm/bridge: panel: Propagate bus format/flags

Propagate bus format/flags so that the previous bridge element in the
chain knows which input format the panel bridge expects.

v11:
* Fix a typo in the subject
* Update the commit message so it's readable by itself

v10:
* Add changelog to the commit message

v8 -> v9:
* No changes

v7:
* Set atomic state hooks explicitly

v4 -> v6:
* Not part of the series

v3:
* Adjust things to match the new bus-format negotiation approach
* Use drm_atomic_helper_bridge_propagate_bus_fmt
* Don't implement ->atomic_check() (the core now takes care of bus
  flags propagation)

v2:
* Adjust things to match the new bus-format negotiation approach

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200128135514.108171-11-boris.brezillon@collabora.com