I really don't know what happend, I just know that I have exactly the same machine of two years ago.

If I launch

systemctl status systemd-backlight@backlight:radeon_bl0.service
I get:

● systemd-backlight@backlight:radeon_bl0.service - Load/Save Screen Backlight Brightness of backlight:radeon_bl0
Loaded: loaded (/usr/lib/systemd/system/systemd-backlight@.service; static; vendor preset: disabled)
Active: active (exited) since ven 2016-04-29 01:38:49 CEST; 56min ago
Docs: man:systemd-backlight@.service(8)
Process: 343 ExecStart=/usr/lib/systemd/systemd-backlight load %i (code=exited, status=0/SUCCESS)
Main PID: 343 (code=exited, status=0/SUCCESS)
-------- EDIT --------

Solved!! smile

I just added acpi_backlight=vendor to the arguments of the kernel and everything seems to work now!

/etc/default/grub:

GRUB_CMDLINE_LINUX_DEFAULT="quiet acpi_backlight=vendor"

dit the file /etc/default/grub and add the below parameters to the GRUB_CMDLINE_LINUX_DEFAULT:

$ sudo nano /etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="quiet acpi_osi=Linux acpi_backlight=vendor"
Do not forget to run sudo update-grub afterwards.