drm/dp_mst: Kill the second sideband tx slot, save the world

While we support using both tx slots for sideband transmissions, it
appears that DisplayPort devices in the field didn't end up doing a very
good job of supporting it. From section 5.2.1 of the DP 2.0
specification:

  There are MST Sink/Branch devices in the field that do not handle
  interleaved message transactions.

  To facilitate message transaction handling by downstream devices, an
  MST Source device shall generate message transactions in an atomic
  manner (i.e., the MST Source device shall not concurrently interleave
  multiple message transactions). Therefore, an MST Source device shall
  clear the Message_Sequence_No value in the Sideband_MSG_Header to 0.

This might come as a bit of a surprise since the vast majority of hubs
will support using both tx slots even if they don't support interleaved
message transactions, and we've also been using both tx slots since MST
was introduced into the kernel.

However, there is one device we'v...