[Fix] Lenovo V570 Screen Brightness

[wjholden]

About a year ago or so I suddenly found myself unable to control the screen brightness on my laptop. I'm not sure what I had changed, nor what took me so long to get around to fixing it. Anyways, if you have a similar laptop with an Intel graphics card, this may help.
I tried a few different things before I settled on these two tweaks, both of which I found at: https://wiki.archlinux.org/index.php/Backlight

• Remove CONFIG_IDEAPAD_LAPTOP from your kernel.
  
• Set acpi_backlight=vendor as a kernel boot parameter. Since I'm using Grub2, this consisted of adding this to GRUB_CMDLINE_LINUX in /etc/default/grub. Commit this change with grub2-mkconfig -o /boot/grub/grub.cfg and reboot.

(Edit: corrected link.)