drm/doc: Add KUnit documentation

Explain how to run the KUnit tests present in the DRM subsystem and
clarify why the UML-only options were not added to the configuration
file present in drivers/gpu/drm/.kunitconfig [1] [2].

[1] https://lore.kernel.org/dri-devel/CABVgOSn8i=LO5p7830h2XU1Jgg0KrN0qTnxkOMhf1oTgxjaKKw@mail.gmail.com/
[2] https://lore.kernel.org/dri-devel/CAGS_qxqpiCim_sy1LDK7PLwVgWf-LKW+uNFTGM=T7ydk-dYcEw@mail.gmail.com/



Reviewed-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: David Gow <davidgow@google.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220620160640.3790-4-jose.exposito89@gmail.com