anv/allocator: Don't shrink either end of the block pool

Previously, we only tried to ensure that we didn't shrink either end
below what was already handed out.  However, due to the way we handle
relocations with block pools, we can't shrink the back end at all.  It's
probably best to not shrink in either direction.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105374
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106147


Tested-by: Eero Tamminen <eero.t.tamminen@intel.com>
Reviewed-by: Scott D Phillips <scott.d.phillips@intel.com>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit 3db93f91)