video-color: Fix for missing clipping in PQ EOTF function

* Add missing clipping in EOTF
* Use pre-calculated values
* Update variable names to specification's defined ones

Part-of: <gstreamer/gstreamer!1629>
26 jobs for !1629 with pq-gamma-decode in 37 minutes and 15 seconds (queued for 2 seconds)
merge request