Bug#1122568: Maybe upstream issue 488

[Jeremy Stanley]

I ran into similar symptoms where the DP/HDMI outputs disappeared 
from all my systems, both this and bug 1122572 look likely 
consistent with https://github.com/alsa-project/alsa-lib/issues/488 
so if someone gets a chance to try packaging the master branch tip 
(or 1.2.15.1 once it's tagged) that might help. In the meantime, 
rolling back to 1.2.14 packages also resolved it for me.