drm/todo: Add entry for converting kselftests to kunit

Many of the kselftests in DRM can be converted to kunit tests instead,
since that framework is more suitable for unit testing.

Suggested-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220509130846.694397-1-javierm@redhat.com