xwayland: Don't take buffer release queue into account for frame timer

The buffer release queue has two kinds of entries:

* Pending async flips.
* Completed flips waiting for their buffer to be released by the Wayland
  compositor.

xwl_present_timer_callback neither completes async flips nor releases
buffers, so the timer isn't needed for the buffer release queue.

(cherry picked from commit e6cd1c9b)