gpu: host1x: Optimize BOs usage when firewall is enabled

We don't need to hold and pin original BOs of the gathers in a case of
enabled firewall because in this case gather's content is copied and the
copy is used by the executed job.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>