drm/atomic: rename async_{update,check} to amend_{update,check}

Asynchronous update is the ability change the hw state at any time, not
only during vblank.

Amend mode is the ability to perform 1000 commits to be applied as soon
as possible without waiting for 1000 vblanks.

async updates can be seen as amend, but the opposite is not true.

&drm_plane_helper_funcs.atomic_async_{update,check}() was being used by
drivers to implement amend and not async. So rename them to amend.

Also improve docs explaining the difference.

If asynchronous is required, normal page flip can be performed using
DRM_MODE_PAGE_FLIP_ASYNC flag.

Signed-off-by: Helen Koike <helen.koike@collabora.com>

Series-to: dri-devel@lists.freedesktop.org
Series-cc: David Airlie <airlied@linux.ie>, linux-kernel@vger.kernel.org, kernel@collabora.com, Stéphane Marchesin <marcheu@google.com>, Tomasz Figa <tfiga@chromium.org>
Series-cc: nicholas.kazlauskas@amd.com, Sean Paul <seanpaul@google.com>, daniel.vetter@ffwll.ch, boris.brezillon@collabora.com, tina.zhang@intel.com
Commit-notes:
Hello,

I would like to officially clarify what async update means by adding it
in the docs.
Please correct me if I am wrong, but the current async_{update,check}
callbacks are being used to do amend (the legacy cursor behavior), i.e.
to allow 1000 updates without waiting for 1000 vblanks.

So I would like to clarify this in the docs and rename the current
callbacks to reflect this behaviour.

I also see that for real async updates, the flag
DRM_MODE_PAGE_FLIP_ASYNC can be used in a normal sync update (it is
already being used by some drivers actually, in the atomic path, not only
in the legacy page flip, at least is what I understood from
amdgpu_dm_atomic_commit_tail() implementation).

Please, let me know if I misunderstood anything. Otherwise renaming and
improving the docs would put us all in sync.

Thanks!
Helen
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