[ Via @sathya-nujella ]
With 3 DP-MST monitors connected to a TGL system, following error is seen:
[ 191.077924] snd_hda_codec_hdmi ehdaudio0D2: HDMI: pin NID 0xa not registered
This bug occur when there are four displays connected (one can be the eDP panel), and three of them connected to DP-MST hub (and all have audio capability).