Arch Linux

(The attached files are from when I was on kernel version 4.9.78-1-lts, but I've confirmed that this bug persists on non-LTS 4.12.12 as well as 4.14.17-1-lts.)

I have two GPUs a GT 740 on nouveau, and an AMD R9 Fury on amdgpu. Both cards are working fine, and both drivers seem to be working fine, other than this issue.

Problem:
DRI3 won't activate, despite no logs that I know of ever spitting out an error message related to it, and despite nothing about my system changing since it last worked other than package versions. I'm not sure exactly when this problem started, because I haven't been playing video games on Linux as often lately (DRI_PRIME is what I need DRI3 for), but it most likely started within the last 3 months.

Steps to reproduce:
I have no idea. This is why I'm stumped. The Xorg conf files are exactly what they should be and exactly what they were the last time this worked. Here are the packages I've tried downgrading to versions that I had when this worked, even all at the same time just in case:
-linux{,-lts}
-xorg-server{,-common,-devel}
-xf86-input-libinput & libinput
-{,lib32-}mesa
-mesa-vdpau
-xf86-video-{nouveau,amdgpu}

I first noticed this problem over 2 weeks ago, but have spent all this time trying to figure out how to reproduce it before doing a bug report, even getting help from #nouveau (and trying to get help from #xorg-users and #xorg-devel, but the former was empty and the latter ignored me). I give up. I can't figure out how to reproduce it unless there are even more packages I can try downgrading that I haven't heard of, but I badly need this fixed.