vp9: Modify logic for gf setting based on up-coming key frame.

For 1 pass vbr mode:
Refactor to move the logic for gf setting based on up-coming
key frames to a separate function, so same logic can be used for
scene-cuts/changes.

Change-Id: Ic4ede308e08ba869bb62e4566e19ea31222c5229
1 file changed