drm/xe: Don't change LRC ring head on job resubmission

drm/xe: Don't change LRC ring head on job resubmission

Now that we save the job's head during submission, it's no longer
necessary to adjust the LRC ring head during resubmission. Instead, a
software-based adjustment of the tail will overwrite the old jobs in
place. For some odd reason, adjusting the LRC ring head didn't work on
parallel queues, which was causing issues in our CI.

v5:
 - Add comment in guc_exec_queue_start explaning why the function works
   (Auld)
v7:
 - Only adjust first state on first unsignaled job (Auld)
v8:
 - Break unsignaled job handling to separate patch (Auld)

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Tomasz Lis <tomasz.lis@intel.com>
Link: https://lore.kernel.org/r/20251008214532.3442967-7-matthew.brost@intel.com