drm/xe: Add SVM VRAM migration

Migration is implemented with range granularity, with VRAM backing being
a VM private TTM BO (i.e., shares dma-resv with VM). The lifetime of the
TTM BO is limited to when the SVM range is in VRAM (i.e., when a VRAM
SVM range is migrated to SRAM, the TTM BO is destroyed).

The design choice for using TTM BO for VRAM backing store, as opposed to
direct buddy allocation, is as follows:

- DRM buddy allocations are not at page granularity, offering no
  advantage over a BO.
- Unified eviction is required (SVM VRAM and TTM BOs need to be able to
  evict each other).
- For exhaustive eviction [1], SVM VRAM allocations will almost certainly
  require a dma-resv.
- Likely allocation size is 2M which makes of size of BO (872)
  acceptable per allocation (872 / 2M == .0004158).

With this, using TTM BO for VRAM backing store seems to be an obvious
choice as it allows leveraging of the TTM eviction code.

Current migration policy is migrate any SVM range greater than or equal
to 64k once.

[1] https://patchwork.freedesktop.org/series/133643/

v2:
 - Rebase on latest GPU SVM
 - Retry page fault on get pages returning mixed allocation
 - Use drm_gpusvm_devmem
v3:
 - Use new BO flags
 - New range structure (Thomas)
 - Hide migration behind Kconfig
 - Kernel doc (Thomas)
 - Use check_pages_threshold
v4:
 - Don't evict partial unmaps in garbage collector (Thomas)
 - Use %pe to print errors (Thomas)
 - Use %p to print pointers (Thomas)
v5:
 - Use range size helper (Thomas)
 - Make BO external (Thomas)
 - Set tile to NULL for BO creation (Thomas)
 - Drop BO mirror flag (Thomas)
 - Hold BO dma-resv lock across migration (Auld, Thomas)
v6:
 - s/drm_info/drm_dbg (Thomas)
 - s/migrated/skip_migrate (Himal)
 - Better debug message on VRAM migration failure (Himal)
 - Drop return BO from VRAM allocation function (Thomas)

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250306012657.3505757-28-matthew.brost@intel.com

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