I ran into this problem again during a fresh install and a @world update while configuring keys for adjusting backlight of the screen. Laptop is a Lenovo X220 with Intel HD 3000. I believe since >=x11-base/xorg-server-1.19.x xbacklight adjustments via keys isn't working anymore. Tehrefore I had to replace x11-apps/xbacklight-1.2.1-r1 with sys-power/acpilight-1.0. Works perfect after adding kernel parameters.

But x11-base/xorg-x11-7.4-r2 keeps pulling x11-apps/xbacklight-1.2.1-r1 which will block sys-power/acpilight-1.0 and keys for adjusting backlights won't work anymore.