drm/atomic-helpers: remove legacy_cursor_update hacks

drm/atomic-helpers: remove legacy_cursor_update hacks

The stuff never really worked, and leads to lots of fun because it
out-of-order frees atomic states. Which upsets KASAN, among other
things.

For async updates we now have a more solid solution with the
->atomic_async_check and ->atomic_async_commit hooks. Support for that
for msm and vc4 landed. nouveau and i915 have their own commit
routines, doing something similar.

For everyone else it's probably better to remove the use-after-free
bug, and encourage folks to use the async support instead. The
affected drivers which register a legacy cursor plane and don't either
use the new async stuff or their own commit routine are: amdgpu,
atmel, mediatek, qxl, rockchip, sti, sun4i, tegra, virtio, and vmwgfx.

Inspired by an amdgpu bug report.

v2: Drop RFC, I think with amdgpu converted over to use
atomic_async_check/commit done in

commit 674e78acae0dfb4beb56132e41cbae5b60f7d662
Author: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Date:   Wed Dec 5 14:59:07 2018 -0500

    drm/amd/display: Add fast path for cursor plane updates

we don't have any driver anymore where we have userspace expecting
solid legacy cursor support _and_ they are using the atomic helpers in
their fully glory. So we can retire this.

v3: Paper over msm and i915 regression. The complete_all is the only
thing missing afaict.

v4: Rebased on recent kernel, added extra link for vc4 bug.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=199425
Link: https://lore.kernel.org/all/20220221134155.125447-9-maxime@cerno.tech/
Cc: mikita.lipski@amd.com
Cc: Michel Dänzer <michel@daenzer.net>
Cc: harry.wentland@amd.com
Cc: Rob Clark <robdclark@gmail.com>
Cc: "Kazlauskas, Nicholas" <nicholas.kazlauskas@amd.com>
Tested-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>