Hi Anton,
Ok, a few more things to try, I have googled around and found an old post on the mintppc forum.
Bear in mind this is from 2012.... This may or may not still be relevant or current info, but give it a go.
http://www.mintppc.org/forums/viewtopic.php?f=15&t=810&start=10
According to this post which is quite interesting, try to disable the second DVI-I-2 port (assuming you are connecting to DVI-I-1).
Which apparently might disable the dvi-2 port from kms auto detection.
Append to yaboot kernel command line:
video=DVI-I-2:d
Run ybin -v after saving yaboot.conf of course.
Which apparently might disable the DVI-I-2 port from kms auto detection.
Again, this is a bit of a hackish workaround, (aren't all workarounds hackish)!
Worth a shot and may help isolate what is going on exactly and possibly give feedback to the nouveau devs to provide a proper patch to the nouveau driver.
This may of course be totally out of date. But an interesting read nonetheless and makes sense!
Info from thread post page 2, 2nd message from 'str8bs'.
Cheers,
Peter