Re-edit /etc/default grub
Goto the line:
And edit to be...
Save, exit, update-grub to pick up the changes...
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=Linux"
Then edit /etc/rc.local
Before the last line that says "exit 0", add a line and insert:
gksudo leafpad /etc/rc.local
Save and exit... Reboot.
setpci -s 00:02.0 F4.B-0
If it still doesn't come up, change /etc/rc.local back (remove the added line). Edit /etc/default/grub and change "acpi_osi=Linux" to "acpi_backlight=vendor"... Save exit, upgdate grub and reboot.
If it doesn't come up again... edit /etc/default/grub and change "acpi_backlight=vendor" to "video.use_bios_initial_backlight=0"... Save exit, upgdate grub and reboot.
And if it still doesn't come up:
And post those results here.
ls /sys/class/backlight/* > ~/backlight.txt