There appears to be a situation where without a known-good display, the firmware will fail to boot to the shell. This also appears to impact the OS in a way I haven't yet identified, but sometimes boot will hang before the getty appears, booting with a known-good display seems to resolve this. This appears to track the display bug I mentioned https://bugs.freedesktop.org/show_bug.cgi?id=75895
Darren reports this is fixed.