drm/rcar-du: Plug atomic state hooks to the default implementation

drm/rcar-du: Plug atomic state hooks to the default implementation

This is needed to pass a bridge state to all atomic hooks, if we don't
do that, the core can't duplicate/create bridge states.

v10:
* Add changelog to the commit message

v9:
* Add Neil's R-b
* Move earlier in the series

v8:
* No changes

v7:
* New patch

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200128135514.108171-3-boris.brezillon@collabora.com