drm/xe: Track page table memory usage for client

Account page table memory usage in the owning client
memory usage stats.

V2:
  - Minor tweak to if (vm->pt_root[id]) check - Himal

Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Signed-off-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>