Open a terminal and create the following configuration file, if it does not exist:

sudo touch /usr/share/X11/xorg.conf.d/20-intel.conf 

Now we need to edit this file. You can use any editor be it a terminal one or graphical.

sudo gedit /usr/share/X11/xorg.conf.d/20-intel.conf        

add the following lines to this file:

Section "Device" Identifier "card0" 
Driver "intel" Option "Backlight" "intel_backlight" 
BusID "PCI:0:2:0" EndSection