(In reply to Darek Deoniziak from comment #13)
[snips]
> Tried updating drivers (using oibaf ppa all the time) and using single file
> with settings:
> Section "Device" 
>         Identifier      "Graphics"
>         Option          "DRI"   "3"
> EndSection
> 
> How can I make sure that it is actually used?

> [     5.034] (II) intel(0): [DRI2] Setup complete
> [     5.034] (II) intel(0): [DRI2]   DRI driver: i965
> [     5.034] (II) intel(0): [DRI2]   VDPAU driver: va_gl
> [     5.034] (II) intel(0): direct rendering: DRI2 enabled

DRI3 would be mentioned there as well (direct rendering: DRI2 DRI3 enabled).
Check your xorg.conf snippet is being loaded. You can check using
xf86-video-intel/tools/dri3info and/or glxinfo.