drm/xe/uapi: Extend the madvise functionality to support foreign pagemap placement for svm

Use device file descriptors and regions to represent pagemaps on
foreign or local devices.

The underlying files are type-checked at madvise time, and
references are kept on the drm_pagemap as long as there is are
madvises pointing to it.

Extend the madvise preferred_location UAPI to support the region
instance to identify the foreign placement.

v2:
- Improve UAPI documentation. (Matt Brost)
- Sanitize preferred_mem_loc.region_instance madvise. (Matt Brost)
- Clarify madvise drm_pagemap vs xe_pagemap refcounting. (Matt Brost)
- Don't allow a foreign drm_pagemap madvise without a fast
  interconnect.
v3:
- Add a comment about reference-counting in xe_devmem_open() and
  remove the reference-count get-and-put. (Matt Brost)

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20251219113320.183860-16-thomas.hellstrom@linux.intel.com

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