Xi: lock the input thread for any pointer barrier list manipulation

The input thread checks the barriers for pointer positioning, swapping the
list out from underneath is considered impolite.

Reported-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
(cherry picked from commit 678d64aa)