radeon_hd: Rework dp aux functions to take connector index

* This is less pretty, but we need access to the connector
  to find the HPD gpio pin mask on the card.
* dp_aux communications seem to work again.
* If you have a DisplayPort item attached to your card you
  may want to just unplug it at this point. We attempt DP
  link training and it fails. This failure will also cause
  other monitors to not function as app_server still isn't
  multi-head aware (#10486)