drm/bridge: Add a drm_bridge_state object

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.

Signed-off-by: Boris Brezillon <[email protected]>
Reviewed-by: Neil Armstrong <[email protected]>
Reviewed-by: Laurent Pinchart <[email protected]>
Signed-off-by: Neil Armstrong <[email protected]>
Reviewed by: Jernej Skrabec <[email protected]>
Tested-by: Jonas Karlman <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]