Comment # 1 on bug 102432 from Gregor Münch

Forget everything, solution:

Ok, the problem is that nobody updated lib32-libdrm in arch. Its still at
2.4.81 so way to old. I installed now the git versions of both packages witch
brings them effectively to 2.4.83 r0 and the problem went away.

In the meantime, I even tried to bisect:

0167e6836e91947418fec36c3b4b396760d0f345 is the first bad commit
commit 0167e6836e91947418fec36c3b4b396760d0f345
Author: Jan Vesely <jan.vesely@rutgers.edu>
Date:   Fri Jul 28 01:46:45 2017 -0400

    amdgpu: Add FX-9800P Bristol Ridge iGPU id

    Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
    Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>

:040000 040000 8b07e8bdce21260ecccb07f424348b838a787472
2c65bba8b27ddf5329fa9f1e3501f33225ca909a M      data

confirmed by reverting, though its pointless.