drm/xe: Rename lookup_vma function to xe_find_vma_by_addr

drm/xe: Rename lookup_vma function to xe_find_vma_by_addr

This update renames the lookup_vma function to xe_vm_find_vma_by_addr and
makes it accessible externally. The function, which looks up a VMA by
its address within a specified VM, will be utilized in upcoming patches.

v2
 - Fix doc

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Acked-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://lore.kernel.org/r/20250513040228.470682-9-himal.prasad.ghimiray@intel.com
Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>