No displayport/hdmi audio with pulseaudio and Linux 5.5

Reported to pulseaudio as:

This started happening with Linux 5.5, with both older patched nvidia drivers and with 440.59. It seems to be due to changes in the audio driver to support displayport MST and probably isn’t directly dependent on the video driver, but I couldn’t say that for sure.

Confirmed and fixed as a kernel bug. Will show up in a stable 5.5.x release.