drm/bridge: Document the drm_encoder.bridge_chain field as private

The drm_encoder.bridge_chain is not meant to be touched manually by
drivers. Make this clear in the documentation.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200226112514.12455-5-laurent.pinchart@ideasonboard.com