glx: drop obsolete _XUnlock_Mutex in __glXInitialize error path

glx: drop obsolete _XUnlock_Mutex in __glXInitialize error path

With commit 1f1928db001(glx: Drop _Xglobal_lock while we create and
initialize glx display) we've split the big _Xglobal_lock handling in
a more fine grained manner.

Unfortunatelly we forgot to drop the unlock_mutex on the error paths,
leading to undefined behaviour as the mutex is already unlocked.

Cc: Kristian Høgsberg <krh@bitplanet.net>
Cc: "9.2 10.0 10.1"  <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>