Just a thought, have you tried with no VGA BIOS? You won't get any screen output until X loads, but after that point it should work fine, while in X. On a plug-in card that's how it works but I don't know if the onboard chip will be enabled by default, enabled by Coreboot or disabled because the VGA BIOS isn't run. I don't know exactly what this might prove but it is something to try anyway.