Bug#495267: Upstream commit which causes this

[Florian Weimer]

* Jed Davis:

>> I guess you'll have to specify a mode as well since just --crtc seems
>> to disable the output.  Try:
>> xrandr --output DVI-0 --off
>> xrandr --output DVI-0 --crtc 1 --mode 1024x768
>
> That does the trick (as does adding the --crtc 1 to the line in my
> .xinitrc that configures that monitor, since that already specified the
> --mode).  Thank you!

Okay, so there is a workaround.  Thanks.  Do you still need more
information to actually fix this regression?