(In reply to Ilia Mirkin from comment #35)
> If I had to put money on one of the left-over commits, I'd go with
> 
> commit 3d42f1ddc47a69c0ce155f9f30d764c4d689a5fa
> Author: Aaron Ma <aaron.ma@canonical.com>
> Date:   Sat Sep 1 02:20:00 2018 +0800
> 
>     vgaarb: Keep adding VGA device in queue
>     
>     If failed to find the deivice owning the boot framebuffer,
>     try to use the first VGA device instead of the last one.
>     
>     Usually the 1st device is integrated GPU who owns the boot framebuffer.
>     
>     Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
>     Acked-by: Alex Deucher <alexander.deucher@amd.com>
>     Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>     Link:
> https://patchwork.freedesktop.org/patch/msgid/1535739600-8842-2-git-send-
> email-aaron.ma@canonical.com
> 
> Which seems to change the vgaarb logic wrt which device is the primary.

Thanks a lot for the hint. I am looking forward to the git bisect result.