tu: Allocate cmd_buffer from its pool

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/14760
Signed-off-by: Karmjit Mahil <karmjit.mahil@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/39623>

Bug: 480139792
Change-Id: I78f6fbfa22a587093e527074f2cd6fba9ecdc190
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/1490111
Reviewed-by: Matt Sandy <msandy@google.com>
Commit-Queue: Craig Stout <cstout@google.com>