clutter/x11/device-manager-xi2: Don't set the stage on enter/leave events

This causes a bug with touch handling in x11 sessions where actors no
longer receive TOUCH_UPDATE events as soon as the touchpoint is moved
above another actor because the stage was set to NULL.
2 jobs for clutter-fix-x11-backend-bug in 6 minutes and 2 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #202422
build-mutter

00:02:23

 
  Test
passed #202423
test-mutter

00:03:38