Open a terminal and type
sudo nano /etc/default/grub
Enter your password. It will not be echoed to the screen.
Look for the line that says
and change it so it says
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor
Press ctrl + o to save and ctrl + x to exit.
Reboot your PC and check your function keys. Post back if it doesn't work.
Edit: Just in case you missed it i have highlighted what you need to enter.