drm/xe: Return first unsignaled job first pending job helper

drm/xe: Return first unsignaled job first pending job helper

In all cases where the first pending job helper is called, we only want
to retrieve the first unsignaled pending job, as this helper is used
exclusively in recovery flows. It is possible for signaled jobs to
remain in the pending list as the scheduler is stopped, so those should
be skipped.

Also, add kernel documentation to clarify this behavior.

v8:
 - Split out into own patch (Auld)

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://lore.kernel.org/r/20251008214532.3442967-6-matthew.brost@intel.com