gpu: host1x: Store pointer to client in jobs

Rather than storing some identifier derived from the application
context that can't be used concretely anywhere, store a pointer to the
client directly so that accesses can be made directly through that
client object.

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