Don't yield for first unlock of sync var for qthreads sync var unlock

Previously yield for if ((lockers_out % 4096) == 0) meant we would yield on the
first unlock. This just changes it to if ((lockers_out % 4096) == 4095) so that
we still yield after the same number of iterations but the first yield is moved
back to the 4095 unlock instead of the first.