drm/tests: helpers: Make sure the device is bound

The device managed resources are freed when the device is detached, so
it has to be bound in the first place.

Let's create a fake driver that we will bind to our fake device to
benefit from the device managed cleanups in our tests.

Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v3-9-4615a663a84a@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>

drivers/gpu/drm/tests/drm_kunit_helpers.c

@@ -18,12 +18,32 @@ struct kunit_dev {
 static const struct drm_mode_config_funcs drm_mode_config_funcs = {
 };
 
+static int fake_probe(struct platform_device *pdev)
+{
+	return 0;
+}
+
+static int fake_remove(struct platform_device *pdev)
+{
+	return 0;
+}
+
+static struct platform_driver fake_platform_driver = {
+	.probe	= fake_probe,
+	.remove	= fake_remove,
+	.driver = {
+		.name	= KUNIT_DEVICE_NAME,
+	},
+};
+
 /**
  * drm_kunit_helper_alloc_device - Allocate a mock device for a KUnit test
  * @test: The test context object
  *
  * This allocates a fake struct &device to create a mock for a KUnit
- * test.
+ * test. The device will also be bound to a fake driver. It will thus be
+ * able to leverage the usual infrastructure and most notably the
+ * device-managed resources just like a "real" device.
  *
  * Callers need to make sure drm_kunit_helper_free_device() on the
  * device when done.
@@ -36,6 +56,9 @@ struct device *drm_kunit_helper_alloc_device(struct kunit *test)
 	struct platform_device *pdev;
 	int ret;
 
+	ret = platform_driver_register(&fake_platform_driver);
+	KUNIT_ASSERT_EQ(test, ret, 0);
+
 	pdev = platform_device_alloc(KUNIT_DEVICE_NAME, PLATFORM_DEVID_NONE);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, pdev);
 
@@ -58,6 +81,7 @@ void drm_kunit_helper_free_device(struct kunit *test, struct device *dev)
 	struct platform_device *pdev = to_platform_device(dev);
 
 	platform_device_unregister(pdev);
+	platform_driver_unregister(&fake_platform_driver);
 }
 EXPORT_SYMBOL_GPL(drm_kunit_helper_free_device);