gpu: host1x: Add job release callback

Add a callback field to the job structure, to be called just before
the job is to be freed. This allows the job's submitter to clean
up any of its own state, like decrement runtime PM refcounts.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>