XIPassiveGrab: Fix completely broken locking in XIGrabTouchBegin

_XIPassiveGrabDevice calls LockDisplay as the first thing it does. That
means that it expects the display to be unlocked. XIGrabTouchBegin locks
the display to check for the XI extension, and then never unlocks it.
Effectively, this meant that anybody that called XIGrabTouchBegin after
XInitThreads just got a deadlock.

Cool.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>