Ok, I got it. Since my git knowledge is quite limited, this "merge" commit is
opening a vast sea of new commits to test. 

I'll dive into bisection using bisect (which actually deals with those merge
commits). I am a bit scared of the amount of commits, may take hours/days.

Please, in the foreseable futur, do not make amd-staging-drm-next lag that
much.

Coming back once I get the faulty _simple_ commit, wish me luck.