drm/xe: Take ref to job's fence in arm

Take ref to job's fence in arm rather than run job. This ref is owned by
the drm scheduler so it makes sense to take the ref before handing over
the job to the scheduler. Also removes an atomic from the run job path.

Suggested-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241021173512.1584248-1-matthew.brost@intel.com