I added "acpi_osi=Linux" to the kernel options and it worked for me. Although dimming when idle doesn't work and the on screen notification doesn't work either.
So I added the also "acpi_backlight
- sudo gedit /etc/default/grub
- find the line starting with GRUB_CMDLINE_LINUX_DEFAULT
- add "acpi_osi=Linux acpi_backlight=vendor" to the options
- sudo update-grub2