Comment 9 for bug 1724933

Thanks Neil! This method works fine. I hope someday somebody will fix it. In any case, I'll leave it here:

Step 1: Fire a terminal (Obviously)

Step 2: Finding the correct module.

Run the following command:
$ ls /sys/class/backlight/
This should list down two modules:
acpi_video0 and intel_backlight
We need to know which module is exactly controlling the brightness.

Run the below command: (You need to be root)

# tee /sys/class/backlight/acpi_video0/brightness <<< 5
If nothing happens, then intel_backlight is the one handling the brightness settings.

Step 3: Modifying the bootloader.

Next step is modifying the file /etc/default/grub.

(Why modify /etc/default/grub and not /boot/grub/grub.cfg)

# nano /etc/default/grub

Insert the below four lines:

GRUB_CMDLINE_LINUX="acpi_backlight=none"
GRUB_CMDLINE_LINUX="acpi_backlight=video"
GRUB_CMDLINE_LINUX="acpi_backlight=vendor"
GRUB_CMDLINE_LINUX="acpi_backlight=native"
After that save and close the file. (Root access needed)

Run (as root):

# update-grub
Step 4: Reboot your system.

Step 5: Verification

After reboot, fire the terminal one last time and

Run the following command:
$ ls /sys/class/backlight/
12
Voila!!!! acpi_video0 is gone!
Check the fn+Brightness keys or change the brightness via the applet. Brightness controls should work just fine.

Additional info: https://wiki.archlinux.org/index.php/Backlight