drm/i915/selftests: add local workqueue for SW fence selftest

Instead of using a global workqueue for the SW fence selftest,
allocate a separate one temporarily only while running the test.

Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/313f4a713053c2b4896ced5b0e9ff456eb85fe57.1686231190.git.jani.nikula@intel.com

Author: lucacoelho

drivers/gpu/drm/i915/selftests/i915_sw_fence.c

@@ -523,12 +523,19 @@ static void task_ipc(struct work_struct *work)
 static int test_ipc(void *arg)
 {
 	struct task_ipc ipc;
+	struct workqueue_struct *wq;
 	int ret = 0;
 
+	wq = alloc_workqueue("i1915-selftest", 0, 0);
+	if (wq == NULL)
+		return -ENOMEM;
+
 	/* Test use of i915_sw_fence as an interprocess signaling mechanism */
 	ipc.in = alloc_fence();
-	if (!ipc.in)
-		return -ENOMEM;
+	if (!ipc.in) {
+		ret = -ENOMEM;
+		goto err_work;
+	}
 	ipc.out = alloc_fence();
 	if (!ipc.out) {
 		ret = -ENOMEM;
@@ -540,7 +547,7 @@ static int test_ipc(void *arg)
 
 	ipc.value = 0;
 	INIT_WORK_ONSTACK(&ipc.work, task_ipc);
-	schedule_work(&ipc.work);
+	queue_work(wq, &ipc.work);
 
 	wait_for_completion(&ipc.started);
 
@@ -563,6 +570,9 @@ static int test_ipc(void *arg)
 	free_fence(ipc.out);
 err_in:
 	free_fence(ipc.in);
+err_work:
+	destroy_workqueue(wq);
+
 	return ret;
 }