drm/ttm: Provide a generic LRU walker helper

Provide a generic LRU walker in TTM, in the spirit of drm_gem_lru_scan()
but building on the restartable TTM LRU functionality.

The LRU walker optionally supports locking objects as part of
a ww mutex locking transaction, to mimic to some extent the
current functionality in ttm. However any -EDEADLK return
is converted to -ENOSPC and then to -ENOMEM before reaching
the driver, so that the driver will need to backoff and possibly retry
without being able to keep the ticket.

v3:
- Move the helper to core ttm.
- Remove the drm_exec usage from it for now, it will be
  reintroduced later in the series.
v4:
- Handle the -EALREADY case if ticketlocking.
v6:
- Some cleanup and added code comments (Matthew Brost)
- Clarified the ticketlock in the commit message (Matthew Brost)
v7:
- Use s64 rather than long for the target and progress
  (Christian König)
- Update documentation to not encourage using pages as a
  progress measure. (Christian König)
- Remove cond_resched(). (Christian König)

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]> #v6
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Signed-off-by: Christian König <[email protected]>

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