drm/ttm: Use LRU hitches

Have iterators insert themselves into the list they are iterating
over using hitch list nodes. Since only the iterator owner
can remove these list nodes from the list, it's safe to unlock
the list and when continuing, use them as a starting point. Due to
the way LRU bumping works in TTM, newly added items will not be
missed, and bumped items will be iterated over a second time before
reaching the end of the list.

The exception is list with bulk move sublists. When bumping a
sublist, a hitch that is part of that sublist will also be moved
and we might miss items if restarting from it. This will be
addressed in a later patch.

Changes in previous series:
- Updated ttm_resource_cursor_fini() documentation.
v2:
- Don't reorder ttm_resource_manager_first() and _next().
  (Christian König).
- Use list_add instead of list_move
  (Christian König)
v3:
- Split into two patches, one cleanup, one new functionality
  (Christian König)
- use ttm_resource_cursor_fini_locked() instead of open-coding
  (Matthew Brost)

Cc: Christian König <[email protected]>
Cc: Somalapuram Amaranath <[email protected]>
Cc: Matthew Brost <[email protected]>
Cc: <[email protected]>
Signed-off-by: Thomas Hellström <[email protected]>
Reviewed-by: Matthew Brost <[email protected]>
Reviewed-by: Christian König <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Signed-off-by: Christian König <[email protected]>

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