anv/allocator: Simplify anv_scratch_pool

anv/allocator: Simplify anv_scratch_pool

The previous implementation was being overly clever and using the
anv_bo::size field as its mutex.  Scratch pool allocations don't happen
often, will happen at most a fixed number of times, and never happen in the
critical path (they only happen in shader compilation).  We can make this
much simpler by just using the device mutex.  This also means that we can
start using anv_bo_init_new directly on the bo and avoid setting fields
one-at-a-time.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Cc: "13.0" <mesa-stable@lists.freedesktop.org>