Timed out waiting for forcewake ?

Upon startup, my laptop will display the message:
doing fast boot
Creating device nodes with udev
[ 2.503996] [drm:__gen6_gt_force_wake_mt_get] *ERROR* Timed out waiting for forcewake old ack to clear.

It then proceeds to prompt for login on a black screen (not the desktop, which I was expecting).

I don't recall changing anything recently that would cause this (though obviously I did). Any ideas on what is causing this and how to fix it?

Any help is much appreciated.