We currently just evict lmem objects to system memory when under memory
pressure. For this case we might lack the usual object mm.pages, which
effectively hides the pages from the i915-gem shrinker, until we
actually "attach" the TT to the object, or in the case of lmem-only
objects it just gets migrated back to lmem when touched again.

For all cases we can just adjust the i915 shrinker LRU each time we also
adjust the TTM LRU. The two cases we care about are:

  1) When something is moved by TTM, including when initially populating
     an object. Importantly this covers the case where TTM moves something from
     lmem <-> smem, outside of the normal get_pages() interface, which
     should still ensure the shmem pages underneath are reclaimable.

  2) When calling into i915_gem_object_unlock(). The unlock should
     ensure the object is removed from the shinker LRU, if it was indeed
     swapped out, or just purged, when the shrinker drops the object lock.

v2(Thomas):
  - Handle managing the shrinker LRU in adjust_lru, where it is always
    safe to touch the object.
v3(Thomas):
  - Pretty much a re-write. This time piggy back off the shrink_pin
    stuff, which actually seems to fit quite well for what we want here.
v4(Thomas):
  - Just use a simple boolean for tracking ttm_shrinkable.
v5:
  - Ensure we call adjust_lru when faulting the object, to ensure the
    pages are visible to the shrinker, if needed.
  - Add back the adjust_lru when in i915_ttm_move (Thomas)
v6(Reported-by: kernel test robot <lkp@intel.com>):
  - Remove unused i915_tt
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com> #v4
Link: https://patchwork.freedesktop.org/patch/msgid/20211018091055.1998191-6-matthew.auld@intel.com