drm/bridge: add transparent bridge helper

Define a helper for creating simple transparent bridges which serve the
only purpose of linking devices into the bridge chain up to the last
bridge representing the connector. This is especially useful for
DP/USB-C bridge chains, which can span across several devices, but do
not require any additional functionality from the intermediate bridges.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>