GLX: Fix a potential deadlock in OnDisplayClosed.

In OnDisplayClosed, unlock the __glXDisplayInfoHash lock before locking
glxContextHashLock.

In CommonMakeCurrent, it holds glxContextHashLock while it tries to take the
__glXDisplayInfoHash lock, so if CommonMakeCurrent and OnDisplayClosed run at
the same time, they could deadlock.

To avoid that, OnDisplayClosed will set a new inTeardown flag in the
__GLXdisplayInfoHash, then it will unlock the display hash before calling
__glXDisplayClosed.

After __glXDisplayClosed is finished, OnDisplayClosed will lock
__glXDisplayInfoHash again long enough to remove the display from the
hashtable.

The inTeardown flag tells __glXLookupDisplay to return NULL, since after that
point, nothing should be trying to look up the display. In principle, we could
just remove the display from the hashtable up front, but then if something did
try to look up the display, then __glXLookupDisplay would try to create a new
__GLXdisplayInfo for it.

Fixes glvnd/libglvnd#213
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