drm/pagemap: Support source migration over interconnect

Support source interconnect migration by using the copy_to_ram() op
of the source device private pages.

Source interconnect migration is required to flush the L2 cache of
the source device, which among other things is a requirement for
correct global atomic operation. It also enables the source GPU to
potentially decompress any compressed content which is not
understood by peers, and finally for the PCIe case, it's expected
that writes over PCIe will be faster than reads.

The implementation can probably be improved by coalescing subregions
with the same source.

v5:
- Update waiting for the pre_migrate_fence and comments around that,
  previously in another patch. (Himal).
- Actually select device private pages to migrate when
  source_peer_migrates is true.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> # For merging through drm-xe.
Link: https://patch.msgid.link/20251219113320.183860-24-thomas.hellstrom@linux.intel.com

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