Adjust the key frame placement condition

The commit removed a limit on key frame detection, which caused a big
drop in all metric measurements for standard HD clip such as _mobcal.

This single change helps two standard HD clips by a huge amount, which
help the overall std-hd set by 2.4% (glb psnr), 0.9% (avg_psnr), 2.1%
(vpxssim).

In the result page:
http://pafr9.prod.google.com:26163/?/cns/rc-d/home/on2-prod/sunkaras/borg-test/yaowu

2012_04_02_1649_yaowu_bugfix_std-hd
2012_04_03_1452_yaowu_hump_std-hd
represent the encoding test results and std-hd set prior and after this
commit respectively.

Change-Id: Ie4313e317c737ea0e699c3a7919c1376744baa1a