drm/xe/madvise: Implement purgeable buffer object support

This allows userspace applications to provide memory usage hints to
the kernel for better memory management under pressure:

Add the core implementation for purgeable buffer objects, enabling
memory reclamation of user-designated DONTNEED buffers during eviction.

This patch implements the purge operation and state machine transitions:

Purgeable States (from xe_madv_purgeable_state):
 - WILLNEED (0): BO should be retained, actively used
 - DONTNEED (1): BO eligible for purging, not currently needed
 - PURGED (2): BO backing store reclaimed, permanently invalid

Design Rationale:
  - Async TLB invalidation via trigger_rebind (no blocking
    xe_vm_invalidate_vma)
  - i915 compatibility: retained field, "once purged always purged"
    semantics
  - Shared BO protection prevents multi-process memory corruption
  - Scratch PTE reuse avoids new infrastructure, safe for fault mode

Note: The madvise_purgeable() function is implemented but not hooked
into the IOCTL handler (madvise_funcs[] entry is NULL) to maintain
bisectability. The feature will be enabled in the final patch when all
supporting infrastructure (shrinker, per-VMA tracking) is complete.

Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Arvind Yadav <arvind.yadav@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260326130843.3545241-4-arvind.yadav@intel.com

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