NVIDIA cards should have set AllowExternalGpus in /usr/share/X11/xorg.conf.d/10-nvidia.conf

I have a Lenovo Thunderbold 3 Graphics Dock (comes integrated NVIDIA MTX 1050 graphics card), see https://support.lenovo.com/de/en/accessories/acc500012

*What happened:*

With nvidia-graphics-drivers-440 I just got a black screen. After looink into the /var/log/Xorg.0.log I noticed:

```

    (WW) NVIDIA(GPU-0): This device is an external GPU, but external GPUs have not

    (WW) NVIDIA(GPU-0): been enabled with AllowExternalGpus. Disabling this device

    (WW) NVIDIA(GPU-0): to prevent crashes from accidental removal.

    (EE) NVIDIA(0): Failing initialization of X screen 0
```

Some googling turned up https://forums.developer.nvidia.com/t/egpu-and-intel-nuc-xorg-conf-issues-login-loop-and-hanging/69409/4

After I added the `Option "AllowExternalGpus" "true"` to `Section "OutputClass"` of `/usr/share/X11/xorg.conf.d/10-nvidia.conf` and rebooting the card worked.

This is a matter for all eGPUs (external GPUs) that run NVIDIA I guess. See reports at https://www.reddit.com/r/thinkpad/comments/fcuhkn/running_thunderbold_3_graphics_dock_nvidia_mtx/ and aforementioned https://forums.developer.nvidia.com/t/egpu-and-intel-nuc-xorg-conf-issues-login-loop-and-hanging/69409/4

*Possible solution*

Add this line to /usr/share/X11/xorg.conf.d/10-nvidia.conf:
```
Section "OutputClass"
    Identifier "nvidia"
    MatchDriver "nvidia-drm"
    Driver "nvidia"
    Option "AllowEmptyInitialConfiguration"
    # vvvvv ADD THIS LINE vvvvvvvvvvvvvvvvvvvvv
    Option "AllowExternalGpus" "true"
    # ^^^^^^^^^ ADD THIS LINE ^^^^^^^^^^^^^^^^^^^
    ModulePath "/usr/lib/x86_64-linux-gnu/nvidia/xorg"
EndSection
```

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: xserver-xorg-video-nvidia-440 440.82+really.440.64-0ubuntu6 [modified: usr/share/X11/xorg.conf.d/10-nvidia.conf]
ProcVersionSignature: Ubuntu 5.4.0-33.37-generic 5.4.34
Uname: Linux 5.4.0-33-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu27.2
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Tue Jun 2 07:30:47 2020
InstallationDate: Installed on 2019-09-19 (257 days ago)
InstallationMedia: Ubuntu 19.04 "Disco Dingo" - Release amd64 (20190416)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: nvidia-graphics-drivers-440
UpgradeStatus: Upgraded to focal on 2020-05-29 (3 days ago)