drm/xe: Merge adjacent default-attribute VMAs during garbage collection

drm/xe: Merge adjacent default-attribute VMAs during garbage collection

While restoring default memory attributes for VMAs during garbage
collection, extend the target range by checking neighboring VMAs. If
adjacent VMAs are CPU-address-mirrored and have default attributes,
include them in the mergeable range to reduce fragmentation and improve
VMA reuse.

v2
-Rebase

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/20251125075628.1182481-3-himal.prasad.ghimiray@intel.com
Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>