drm/xe/svm: Enable UNMAP for VMA merging operations

drm/xe/svm: Enable UNMAP for VMA merging operations

ALLOW UNMAP of VMAs associated with SVM mappings when the MAP operation
is intended to merge adjacent CPU_ADDR_MIRROR VMAs.

v2
- Remove mapping exist check in garbage collector

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/20251125075628.1182481-5-himal.prasad.ghimiray@intel.com
Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>