The current threshold is knid of low, and in many cases NEWMV
mode is checked but not picked as the best mode. This patch
added a speed feature to increase NEWMV threshold, so that
less partition mode checking goes to check NEWMV. This feature
is enabled for speed 6 and 7.

Rtc set borg tests showed:
1. Speed 6, overall psnr: -0.088%, ssim: -1.339%;
   Average speedup on rtc set is 11.1%.
2. Speed 7, overall psnr: -0.505%, ssim: -2.320%
   Average speedup on rtc set is 12.9%.

Change-Id: I953b849eeb6e0d5a1f13eacba30c14204472c5be