i915 ttm_tt shmem backend

[Matthew Auld]

Hi Christian,

We are looking into using shmem as a ttm_tt backend in i915 for cached
system memory objects. We would also like to make such objects visible
to the i915-gem shrinker, so that they may be swapped out or discarded
when under memory pressure.

One idea for handling this is roughly something like:
- Add a new TTM_PAGE_FLAG_SHMEM flag, or similar.
- Skip the ttm_pages_allocated accounting on such objects, similar to
how FLAG_SG is already handled.
- Skip all the page->mapping and page->index related bits, like in
tt_add_mapping, since it looks like these are set and used by shmem.
Not sure what functionally this might break, but looks like it's maybe
only driver specific?
- Skip calling into ttm_bo_swap_out/in and just have
ttm_populate/unpopulate handle this directly for such objects.
- Make such objects visible to the i915-gem shrinker.

Does this approach look acceptable?