Attach a top-level bridge to each encoder, which will be used for
negociating the bus format and flags.

All the bridges are now attached with DRM_BRIDGE_ATTACH_NO_CONNECTOR.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20211026181240.213806-7-paul@crapouillou.netTested-by: Nikolaus Schaller <hns@goldelico.com>
Reviewed-by: Christophe Branchereau <cbranchereau@gmail.com>