drm/xe: Fix unbind of unaccessed VMA (fault mode)

In fault mode, page table binding is deferred until fault handler.
Thus vma->tile_present will be unset unless the VMA is accessed by GPU.

During a later unbind, the logic doesn't account for the fact that local
fence variable will be NULL in this case, leading to pass NULL into
dma_fence_add_callback() and causing few WARN_ONs to print to console.
The fix is already present in the code, just hoist the fence variable
computation to be done earlier.

Resolves warnings seen with igt@xe_exec_fault_mode@once-invalid-fault
Signed-off-by: Brian Welty <brian.welty@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/xe_vm.c

@@ -1740,14 +1740,14 @@ xe_vm_unbind_vma(struct xe_vma *vma, struct xe_exec_queue *q,
 		}
 	}
 
+	fence = cf ? &cf->base : !fence ?
+		xe_exec_queue_last_fence_get(wait_exec_queue, vm) : fence;
 	if (last_op) {
 		for (i = 0; i < num_syncs; i++)
-			xe_sync_entry_signal(&syncs[i], NULL,
-					     cf ? &cf->base : fence);
+			xe_sync_entry_signal(&syncs[i], NULL, fence);
 	}
 
-	return cf ? &cf->base : !fence ?
-		xe_exec_queue_last_fence_get(wait_exec_queue, vm) : fence;
+	return fence;
 
 err_fences:
 	if (fences) {