anv: Don't place scratch buffers above the 32-bit boundary

anv: Don't place scratch buffers above the 32-bit boundary

This fixes rendering corruptions in DOOM.  Hopefully, it will also make
Jenkins a bit more stable as we've been seeing some random failures and
GPU hangs ever since turning on 48bit.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100620
Fixes: 651ec926fc1 "anv: Add support for 48-bit addresses"
Tested-by: Grazvydas Ignotas <notasas@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: "17.1" <mesa-stable@lists.freedesktop.org>