drm/todo: Add some cleanup tasks

  leftovers from older (never merged into upstream) KMS designs where modes

  where set using their ID, including support to add/remove modes.

- Make ->funcs and ->helper_private vtables optional. There's a bunch of empty

  function tables in drivers, but before we can remove them we need to make sure

  that all the users in helpers and drivers do correctly check for a NULL

  vtable.

- Cleanup up the various ->destroy callbacks. A lot of them just wrapt the

  drm_*_cleanup implementations and can be removed. Some tack a kfree() at the

  end, for which we could add drm_*_cleanup_kfree(). And then there's the (for

  historical reasons) misnamed drm_primary_helper_destroy() function.

Better Testing

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