drm/bridge: Rename bridge helpers targeting a bridge chain

Change the prefix of bridge helpers targeting a bridge chain from
drm_bridge_ to drm_bridge_chain_ to better reflect the fact that
the operation will happen on all elements of chain, starting at the
bridge passed in argument.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191203141515.3597631-2-boris.brezillon@collabora.com