drm/bridge: Make the bridge chain a double-linked list

drm/bridge: Make the bridge chain a double-linked list

So that each element in the chain can easily access its predecessor.
This will be needed to support bus format negotiation between elements
of the bridge chain.

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