drm/xe: Fix a NULL vs IS_ERR() in xe_vm_add_compute_exec_queue()

The xe_preempt_fence_create() function returns error pointers.  It
never returns NULL.  Update the error checking to match.

Fixes: dd08ebf ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/aJTMBdX97cof_009@stanley.mountain
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
(cherry picked from commit 75cc23f)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

Author: Dan Carpenter

drivers/gpu/drm/xe/xe_vm.c

@@ -240,8 +240,8 @@ int xe_vm_add_compute_exec_queue(struct xe_vm *vm, struct xe_exec_queue *q)
 
 	pfence = xe_preempt_fence_create(q, q->lr.context,
 					 ++q->lr.seqno);
-	if (!pfence) {
-		err = -ENOMEM;
+	if (IS_ERR(pfence)) {
+		err = PTR_ERR(pfence);
 		goto out_fini;
 	}