drm/bridge: Add the drm_bridge_get_prev_bridge() helper

The drm_bridge_get_prev_bridge() helper will be useful for bridge
drivers that want to do bus format negotiation with their neighbours.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191203141515.3597631-7-boris.brezillon@collabora.com