kmssink: Renders at half my display refresh rate

Submitted by Nicolas Dufresne `@ndufresne`

While doing some latency test, I notice that kmssink renders at half the speed of my display. My test pipeline was:

gst-launch-1.0 -v videotestsrc ! video/x-raw,framerate=60/1 ! fpsdisplaysink text-overlay=0 video-sink=kmssink

Commenting out the code that (according to comment) tries to wait for previous frame to finish fixes it.

My impression is that we endup waiting for an extra vblank all the time as the previous blank is behind.