drm/i915/gem: Implement object migration

Introduce an interface to migrate objects between regions.
This is primarily intended to migrate objects to LMEM for display and
to SYSTEM for dma-buf, but might be reused in one form or another for
performance-based migration.

v2:
- Verify that the memory region given as an id really exists.
  (Reported by Matthew Auld)
- Call i915_gem_object_{init,release}_memory_region() when switching region
  to handle also switching region lists. (Reported by Matthew Auld)
v3:
- Fix i915_gem_object_can_migrate() to return true if object is already in
  the correct region, even if the object ops doesn't have a migrate()
  callback.
- Update typo in commit message.
- Fix kerneldoc of i915_gem_object_wait_migration().
v4:
- Improve documentation (Suggested by Mattew Auld and Michael Ruhl)
- Always assume TTM migration hits a TTM move and unsets the pages through
  move_notify. (Reported by Matthew Auld)
- Add a dma_fence_might_wait() annotation to
  i915_gem_object_wait_migration() (Suggested by Daniel Vetter)
v5:
- Re-add might_sleep() instead of __dma_fence_might_wait(), Sent
  v4 with the wrong version, didn't compile and __dma_fence_might_wait()
  is not exported.
- Added an R-B.

Reported-by: kernel test robot <[email protected]>
Signed-off-by: Thomas Hellström <[email protected]>
Reviewed-by: Michael J. Ruhl <[email protected]>
Reviewed-by: Matthew Auld <[email protected]>
Signed-off-by: Matthew Auld <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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