drm/ttm: Fixed a read/write lock imbalance

In ttm_write_lock(), the uninterruptible path should call
__ttm_write_lock() not __ttm_read_lock().  This fixes a vmwgfx hang
on F23 start up.

syeh: Extracted this from one of Thomas' internal patches.

Cc: <stable@vger.kernel.org>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>

drivers/gpu/drm/ttm/ttm_lock.c

@@ -180,7 +180,7 @@ int ttm_write_lock(struct ttm_lock *lock, bool interruptible)
 			spin_unlock(&lock->lock);
 		}
 	} else
-		wait_event(lock->queue, __ttm_read_lock(lock));
+		wait_event(lock->queue, __ttm_write_lock(lock));
 
 	return ret;
 }