On my Intel Compute Stick STCK1 (baytrail hdmi audio) 
sound is not working with the kernel 5.6

It doesn't seem an alsa issue. The output of alsa-info.sh is essentially the
same when booting 5.5 or 5.6.

I have bisected the kernel and I found the commit that introduced the issue:

commit 58d124ea2739e1440ddd743d46c470fe724aca9a
Author: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Date:   Thu Oct 31 12:26:04 2019 +0100

    drm/i915: Complete crtc hw/uapi split, v6.

    Now that we separated everything into uapi and hw, it's
    time to make the split definitive. Remove the union and
    make a copy of the hw state on modeset and fastset.

    Color blobs are copied in crtc atomic_check(), right
    before color management is checked.

I'm contacting intel-gfx to let them know about the problem.