drm/xe: Add helper to extend CPU-mirrored VMA range for merge

drm/xe: Add helper to extend CPU-mirrored VMA range for merge

Introduce xe_vm_find_cpu_addr_mirror_vma_range(), which computes an
extended range around a given range by including adjacent VMAs that are
CPU-address-mirrored and have default memory attributes. This helper is
useful for determining mergeable range without performing the actual merge.

v2
- Add assert
- Move unmap check to this patch

v3
- Decrease offset to check by SZ_4K to avoid wrong vma return in fast
  lookup path

v4
- *start should be >= SZ_4K (Matt)

Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20251125075628.1182481-2-himal.prasad.ghimiray@intel.com
Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>