Comment # 3 on bug 1019337 from Takashi Iwai

Boot with drm.debug=0x0e boot option and give the kernel message (dmesg
output).  It'll show whether i915 driver detects its backlight control.

The ACPI provides multiple backlight control, and one of them might actually
work.  Check /sys/class/backlight/*.  You can change the value directly by
writing to "brightness" file (up to the max value found in "max_brightness"),
e.g.
   echo -n 10 > /sys/class/backlight/xxx/brightness

Also, if none of them works, try to pass acpi_backlight=vendor boot option,
too.