drm/doc: Document KUnit expectations

drm/doc: Document KUnit expectations

The DRM and KMS frameworks and helpers gain more and more kunit
coverage, so let's document what our expectations are.

Suggested-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Maíra Canal <mcanal@igalia.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20250113101100.1373856-1-mripard@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>