drm/tests: Add Kunit Helpers

As the number of kunit tests in KMS grows further, we start to have
multiple test suites that, for example, need to register a mock DRM
driver to interact with the KMS function they are supposed to test.

Let's add a file meant to provide those kind of helpers to avoid
duplication.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Tested-by: Mateusz Kwiatkowski <kfyatek+publicgit@gmail.com>
Link: https://lore.kernel.org/r/20220728-rpi-analog-tv-properties-v9-2-24b168e5bcd5@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>