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.tech


Signed-off-by: Maxime Ripard <maxime@cerno.tech>