Relied on an AUR package for building and signing my unified kernel image… one day it was outdated and geberating the image failed, I noticed that by the fact that the system refused to boot my OS. Fixing it was done in a few minutes but boy, that was a shock :D
Guess who also checks the exact output of the kernel rebuild now before rebooting!
Did you take a little journey into the BIOS yet? Is definitely firmware, the question is if you can just change a setting there. Otherwise, somebody already mentioned dummy HDMI plugs.