drm/tests: helpers: Add atomic helpers

The mock device we were creating was missing any of the driver-wide
helpers. That was fine before since we weren't testing the atomic state
path, but we're going to start, so let's use the default
implementations.
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240222-kms-hdmi-connector-state-v7-2-8f4af575fce2@kernel.org

drivers/gpu/drm/tests/drm_kunit_helpers.c

 // SPDX-License-Identifier: GPL-2.0
 
 #include <drm/drm_atomic.h>
+#include <drm/drm_atomic_helper.h>
 #include <drm/drm_drv.h>
 #include <drm/drm_kunit_helpers.h>
 #include <drm/drm_managed.h>
@@ -14,6 +15,8 @@
 #define KUNIT_DEVICE_NAME	"drm-kunit-mock-device"
 
 static const struct drm_mode_config_funcs drm_mode_config_funcs = {
+	.atomic_check	= drm_atomic_helper_check,
+	.atomic_commit	= drm_atomic_helper_commit,
 };
 
 /**