drm/bridge: Introduce drm_bridge_get_next_bridge()

[ Upstream commit fadf872d9d9274a3be34d8438e0f6bb465c8f98b ]

And use it in drivers accessing the bridge->next field directly.
This is part of our attempt to make the bridge chain a double-linked list
based on the generic list helpers.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191203141515.3597631-3-boris.brezillon@collabora.com


Stable-dep-of: 13fcfcb2a9a4 ("drm/msm/mdp5: Add check for kzalloc")
Signed-off-by: Sasha Levin <sashal@kernel.org>