gpu: host1x: Do not leak BO's phys address to userspace

Perform gathers coping before patching them, so that original gathers are
left untouched. That's not as bad as leaking kernel addresses, but still
doesn't feel right.

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