As far as the issue about desktop corruption and lockup on login requiring the
AMD_DEBUG=nodcc workaround:

b563460b494e9228cf5bb1aa4a70ac2499ad81fe is the first bad commit
commit b563460b494e9228cf5bb1aa4a70ac2499ad81fe
Author: Marek Olšák <marek.olsak@amd.com>
Date:   Tue Jan 8 20:08:08 2019 -0500

    radeonsi: enable displayable DCC on Ravens

 src/amd/common/ac_gpu_info.c                      | 8 ++++++++
 src/amd/vulkan/winsys/amdgpu/radv_amdgpu_winsys.c | 4 ++++
 2 files changed, 12 insertions(+)