drm/xe: Take job list lock in xe_sched_add_pending_job

A fragile micro optimization in xe_sched_add_pending_job relied on both
the GPU scheduler being stopped and fence signaling stopped to safely
add a job to the pending list without the job list lock in
xe_sched_add_pending_job. Remove this optimization and just take the job
list lock.

Fixes: 7ddb9403 ("drm/xe: Sample ctx timestamp to determine if jobs have timed out")
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241003001657.3517883-2-matthew.brost@intel.com