drm/xe: Enable 2M pages in xe_migrate_vram

drm/xe: Enable 2M pages in xe_migrate_vram

Using 2M pages in xe_migrate_vram has two benefits: we issue fewer
instructions per 2M copy (1 vs. 512), and the cache hit rate should be
higher. This results in increased copy engine bandwidth, as shown by
benchmark IGTs.

Enable 2M pages by reserving PDEs in the migrate VM and using 2M pages
in xe_migrate_vram if the DMA address order matches 2M.

v2:
 - Reuse build_pt_update_batch_sram (Stuart)
 - Fix build_pt_update_batch_sram for PAGE_SIZE > 4K
v3:
 - More fixes for PAGE_SIZE > 4K, align chunk, decrement chunk as needed
 - Use stack incr var in xe_migrate_vram_use_pde (Stuart)
v4:
 - Split PAGE_SIZE > 4K fix out in different patch (Stuart)

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Stuart Summers <stuart.summers@intel.com>
Link: https://lore.kernel.org/r/20251013034555.4121168-3-matthew.brost@intel.com