Am I running into this thing?
ticket.coreboot.org/issues/453
"Intel HDMI / DP Audio device not showing up after libgfxinit"
I don't think I am, it sounds like I shouldn't be seeing the device at all in that case.
But still, it does make me think that this could be a coreboot/libreboot issue and that's why nobody is talking about it in forums and Q&A sites.