Did the same as Dan, i.e built the kernel (apt-get install source) with the upstream proposed patch 0001-drm-amdgpu-fix-sdma-doorbell-init-ordering-on-APUs.patch using the mentioned test-patches script. The kernel really builds, is even installable, but then hangs as before at boot, as Dan reported before. However, i'm even less experienced kernel builder than Dan (my 1st kernel build ever)...