eg/compute: Use reference counting to handle compute memory pool.

Use pipe_reference to release old RAT surfaces.
RAT surface adds a reference to pool bo, so use reference counting for pool->bo
as well.

v2: Use the same pattern for both defrag paths
    Drop confusing comment

CC: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit f3521ce2)