drm/xe/ufence: Warn if mmget_not_zero() fails

This shouldn't happen but seen this while debugging ufence timeout
issue time to time so log it to isolate this particular case.

v2: s/XE_WARN_ON/drm_dbg(Maarten)

Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/1630
Cc: Francois Dugast <francois.dugast@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241016082304.66009-3-nirmoy.das@intel.com
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>

Author: nirmoy

drivers/gpu/drm/xe/xe_sync.c

@@ -83,6 +83,8 @@ static void user_fence_worker(struct work_struct *w)
 			XE_WARN_ON("Copy to user failed");
 		kthread_unuse_mm(ufence->mm);
 		mmput(ufence->mm);
+	} else {
+		drm_dbg(&ufence->xe->drm, "mmget_not_zero() failed, ufence wasn't signaled\n");
 	}
 
 	wake_up_all(&ufence->xe->ufence_wq);