drm/tests: helpers: Move the helper header to include/drm

We'll need to use those helpers from drivers too, so let's move it to a
more visible location.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v3-1-4615a663a84a@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>

drivers/gpu/drm/tests/drm_kunit_helpers.c

@@ -1,14 +1,13 @@
 // SPDX-License-Identifier: GPL-2.0
 
 #include <drm/drm_drv.h>
+#include <drm/drm_kunit_helpers.h>
 #include <drm/drm_managed.h>
 
 #include <kunit/resource.h>
 
 #include <linux/device.h>
 
-#include "drm_kunit_helpers.h"
-
 struct kunit_dev {
 	struct drm_device base;
 };