Simplify 2 pass KF bitrate allocation
Simplify 2 pass KF bitrate allocation

Simplify the calculation of KF bitrate in similar way
to previous patch for GF/arf.

This has no impact on derf or std hd sets but gives a
small net gain of ~0.1% for yt and yt-hd sets.

Change-Id: Ida64ac1428d9c2a62adb67056fadbf0180eff030

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