Fix comment about the OUT_FENCE timestamp
The timestamp of the KMS commit out-fence is not the same as the completion timestamp passed to the event handler. Many drivers support "high-precision" timestamps, where the timestamp passed to the handler is corrected by the hardware to some implementation-dependent point in the vblank sequence. The timestamp of the out-fence, however, is closer to the time of the display hardware interrupt.
Simply drop the comment there for now.