Here is a new test kernel which not only revert the quirk, but introduce a new 
way to quirk the panels which support DPCD backlight
https://people.canonical.com/~acelan/bugs/lp1861521/

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https://bugs.launchpad.net/bugs/1858844

Title:
  [Dell XPS 15 9575] Screen brightness stopped working with linux-
  image-5.4.0-9-generic

Status in linux package in Ubuntu:
  Confirmed

Bug description:
  I upgraded to Focal Fossa 3 days ago and nothing broke. At the time,
  it was still using linux 5.3 (linux-image-5.3.0-24-generic).

  Today, it updated to linux 5.4 (linux-image-5.4.0-9-generic) and after
  rebooting, the screen brightness does not work anymore - neither from
  keyboard keys nor from the slider in the gnome-shell panel.

  It works again when rebooting into the previous linux-
  image-5.3.0-24-generic kernel.

  ProblemType: Bug
  DistroRelease: Ubuntu 20.04
  Package: linux-image-5.4.0-9-generic 5.4.0-9.12
  ProcVersionSignature: Ubuntu 5.4.0-9.12-generic 5.4.3
  Uname: Linux 5.4.0-9-generic x86_64
  ApportVersion: 2.20.11-0ubuntu15
  Architecture: amd64
  AudioDevicesInUse:
   USER        PID ACCESS COMMAND
   /dev/snd/controlC0:  mario      1892 F.... pulseaudio
  CurrentDesktop: ubuntu:GNOME
  Date: Wed Jan  8 20:23:31 2020
  InstallationDate: Installed on 2018-09-13 (481 days ago)
  InstallationMedia: Ubuntu 18.10 "Cosmic Cuttlefish" - Alpha amd64 (20180912)
  MachineType: Dell Inc. XPS 15 9575
  ProcFB: 0 i915drmfb
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-5.4.0-9-generic 
root=UUID=26ff4c95-5f68-4121-b7d0-989fa705fcc8 ro quiet splash
  RelatedPackageVersions:
   linux-restricted-modules-5.4.0-9-generic N/A
   linux-backports-modules-5.4.0-9-generic  N/A
   linux-firmware                           1.184
  SourcePackage: linux
  UpgradeStatus: Upgraded to focal on 2020-01-04 (3 days ago)
  dmi.bios.date: 07/07/2019
  dmi.bios.vendor: Dell Inc.
  dmi.bios.version: 1.7.1
  dmi.board.name: 0C32VW
  dmi.board.vendor: Dell Inc.
  dmi.board.version: A00
  dmi.chassis.type: 31
  dmi.chassis.vendor: Dell Inc.
  dmi.modalias: 
dmi:bvnDellInc.:bvr1.7.1:bd07/07/2019:svnDellInc.:pnXPS159575:pvr:rvnDellInc.:rn0C32VW:rvrA00:cvnDellInc.:ct31:cvr:
  dmi.product.family: XPS
  dmi.product.name: XPS 15 9575
  dmi.product.sku: 080D
  dmi.sys.vendor: Dell Inc.

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