Use FAST_HEX in speed 5 and 6, which covers more points than
FAST_DIAMOND and improves motion search quality.

At speed 6, RTC set borg tests showed slight quality gain (psnr
gain: 0.143%, ssim gain: 0.226%). No noticeable encoding speed
change.

Change-Id: Ifa62875d9a52ee382ec494f271382bb77d8c67bf