Bisected to
commit 2769dadb0fafdbafc98630fdf96924a3bb209ab7
Author: Marek Olšák <marek.olsak@amd.com>
Date:   Thu Apr 13 23:46:59 2017 +0200

    gallium/radeon: always flush asynchronously and wait after begin_new_cs

    This hides the overhead of everything in the driver after the CS flush and
    before returning from pipe_context::flush.
    Only microbenchmarks will benefit.

    +2% FPS for glxgears.

    Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>

Reverting this commit on the recent git tree fixes the freezes.
Now since the bug doesn't always appear, I'm not 100% sure, but I didn't get a
single freeze in 20 runs, while I got 13 out of 20 without reverting it.

Could you maybe look into this? I have no idea how to fix the issue without
throwing away all changes from this commit.