Ubuntu – Reverting to nouveau in ubuntu 18.04 doesn’t work as expected

Since I had driver problems with nouveau on my Thinkpad (flickering external monitor on docking station) I switched to nvidia-390 and also to nvidia-396 both without success (didn't even recognize the external screen on docking station).

Now I want to switch back to nouveau since then at least the external monitor worked if I connect it via mini-dp.

However I cannot switch back anymore. I did the following steps:

• sudo apt-get purge '*-nvidia-*'
• removed /etc/modprobe.d/blacklist-nouveau.conf (which I created when installing nvidia)
• sudo update-initramfs -u
• sudo reinstall xserver-xorg-core libgl1-mesa-glx xserver-xorg-video-nouveau
• There was no /etc/X11/xorg.conf to remove or to modify
• reboot

However then lspci -nnk | grep -iA2 vga
returns that

00:02.0 VGA compatible controller [0300]: Intel Corporation 3rd Gen Core processor Graphics Controller [8086:0166] (rev 09)
    Subsystem: Lenovo 3rd Gen Core processor Graphics Controller [17aa:21f5]
    Kernel driver in use: i915
--
01:00.0 VGA compatible controller [0300]: NVIDIA Corporation GK107GLM [Quadro K2000M] [10de:0ffb] (rev a1)
    Subsystem: Lenovo GK107GLM [Quadro K2000M] [17aa:21f5]
    Kernel modules: nvidiafb, nouveau

I.e. additionally to nouveau there seems to be nvidiafb active. Then I tried to blacklist nvidiafb without success.

I also tried to disable framebuffer as described in the arch wiki also without success.

The problem is that this configuration doesn't recognize any external monitor even on mini-dp. I just realized that even VGA doesn't work.

Any idea of how I can make it work?

Are there any settings or files I should check to diagnose the cause of the problem?