drm/xe: Do not dereference NULL job->fence in trace points

job->fence is not assigned until xe_sched_job_arm(), check for
job->fence in xe_sched_job_seqno() so any usage of this function (trace
points) do not result in NULL ptr dereference. Also check job->fence
before assigning error in job trace points.

Fixes: 0ac7a2c7 ("drm/xe: Don't initialize fences at xe_sched_job_create()")
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240605055041.2082074-1-matthew.brost@intel.com

drivers/gpu/drm/xe/xe_sched_job.h

@@ -70,7 +70,7 @@ to_xe_sched_job(struct drm_sched_job *drm)
 
 static inline u32 xe_sched_job_seqno(struct xe_sched_job *job)
 {
-	return job->fence->seqno;
+	return job->fence ? job->fence->seqno : 0;
 }
 
 static inline u32 xe_sched_job_lrc_seqno(struct xe_sched_job *job)

drivers/gpu/drm/xe/xe_trace.h

@@ -270,7 +270,7 @@ DECLARE_EVENT_CLASS(xe_sched_job,
 			   __entry->guc_state =
 			   atomic_read(&job->q->guc->state);
 			   __entry->flags = job->q->flags;
-			   __entry->error = job->fence->error;
+			   __entry->error = job->fence ? job->fence->error : 0;
 			   __entry->fence = job->fence;
 			   __entry->batch_addr = (u64)job->ptrs[0].batch_addr;
 			   ),