ac/radeonsi: fix CIK copy max size

While adding transfer queues to radv, I started writing some tests,
the first test I wrote fell over copying a buffer larger than this
limit.

Checked AMDVLK and found the correct limit.

Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 2c1f249f)