drm/tests: helpers: Add SPDX header

The SPDX header is missing, let's add it and fix the corresponding
checkpatch warning.
Suggested-by: Maíra Canal <mairacanal@riseup.net>
Fixes: 44a39283 ("drm/tests: Add Kunit Helpers")
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Link: https://lore.kernel.org/r/20221116151833.1679379-2-maxime@cerno.techSigned-off-by: Maxime Ripard <maxime@cerno.tech>

drivers/gpu/drm/tests/drm_kunit_helpers.c

+// SPDX-License-Identifier: GPL-2.0
+
 #include <drm/drm_drv.h>
 #include <drm/drm_managed.h>
 

drivers/gpu/drm/tests/drm_kunit_helpers.h

+// SPDX-License-Identifier: GPL-2.0
+
 #ifndef DRM_KUNIT_HELPERS_H_
 #define DRM_KUNIT_HELPERS_H_