drm/xe: Only zap PTEs as needed

If PTEs are already invalidated no need to invalidate again.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240514232325.84508-1-matthew.brost@intel.com