Created attachment 142574 [details] [review]
0001-drm-amd-display-Add-fast-path-for-legacy-cursor-plan.patch

This patch is similar to the async_update one but it takes care to lock if
anything is modifying the plane. It's very close to what i915 does with a few
minor differences with framebuffer handling.

I've tested it for compton with Gallium HUD up and I no longer see the issue on
mouse movement (cursor fb changes are still a bit slow, so you'll still
probably see spikes on cursor changes).

You can try this on top of amd-staging-drm-next and I'd imagine it'd fix your
problems.