So as a wokraround:

sudo vim /etc/default/grub

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
#GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

sudo update-grub

reboot

This option adds the dell_backlight:
ubuntu@ubuntu-Latitude-E6430:~$ ls /sys/class/backlight
dell_backlight  intel_backlight  nv_backlight

Note that sending events to the intel_backlight like so:
root@ubuntu-Latitude-E6430:~# echo 1000 > 
/sys/class/backlight/intel_backlight/brightness 
works too.

More info regarding backlight troubleshooting:
http://hansdegoede.livejournal.com/13889.html

I guess we need a kernel patch/quirk.

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You received this bug notification because you are a member of Kernel
Packages, which is subscribed to linux in Ubuntu.
https://bugs.launchpad.net/bugs/1683445

Title:
  E6430 brightness control not working

Status in linux package in Ubuntu:
  New

Bug description:
  The Dell E6430 with Nvidia Optimus enables in BIOS, using the Open
  Source Nouveau driver shows the brightness slider moving but does not
  affect the brightness.

  When using the kernel boot option acpi_backlight=vendor , the
  brightness control works.

  ProblemType: Bug
  DistroRelease: Ubuntu 17.04
  Package: linux-image-4.10.0-19-generic 4.10.0-19.21
  ProcVersionSignature: Ubuntu 4.10.0-19.21-generic 4.10.8
  Uname: Linux 4.10.0-19-generic x86_64
  NonfreeKernelModules: wl
  ApportVersion: 2.20.4-0ubuntu4
  Architecture: amd64
  AudioDevicesInUse:
   USER        PID ACCESS COMMAND
   /dev/snd/controlC0:  ubuntu     2081 F.... pulseaudio
  CurrentDesktop: Unity:Unity7
  Date: Mon Apr 17 20:00:56 2017
  HibernationDevice: RESUME=UUID=66528b53-0f42-4043-9ff8-da8f86036be6
  InstallationDate: Installed on 2017-04-10 (7 days ago)
  InstallationMedia: Ubuntu 17.04 "Zesty Zapus" - Beta amd64 (20170321)
  MachineType: Dell Inc. Latitude E6430
  ProcFB:
   0 nouveaufb
   1 inteldrmfb
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.10.0-19-generic.efi.signed 
root=UUID=4e08c33d-f8cf-4159-a559-a0463d67b96c ro quiet splash 
acpi_backlight=vendor vt.handoff=7
  RelatedPackageVersions:
   linux-restricted-modules-4.10.0-19-generic N/A
   linux-backports-modules-4.10.0-19-generic  N/A
   linux-firmware                             1.164
  SourcePackage: linux
  UpgradeStatus: No upgrade log present (probably fresh install)
  dmi.bios.date: 01/18/2016
  dmi.bios.vendor: Dell Inc.
  dmi.bios.version: A18
  dmi.board.name: 0H3MT5
  dmi.board.vendor: Dell Inc.
  dmi.board.version: A00
  dmi.chassis.type: 9
  dmi.chassis.vendor: Dell Inc.
  dmi.modalias: 
dmi:bvnDellInc.:bvrA18:bd01/18/2016:svnDellInc.:pnLatitudeE6430:pvr01:rvnDellInc.:rn0H3MT5:rvrA00:cvnDellInc.:ct9:cvr:
  dmi.product.name: Latitude E6430
  dmi.product.version: 01
  dmi.sys.vendor: Dell Inc.

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