Adds speed 8 to vp9 as reference

Adds a speed 8 to VP9 where only the nearestmv (0 mv) is searched.
This seems to be about the same speed as vp8 speed 5.

Adds a new speed feature to disable inter modes based on a mask for
each blocksize.

Adds code for having lower complexity motion search methods
in nonrd pick mode function, even though speed 7 still uses DIAMOND
search for now.

Also uses HEX search for speed 6 rather than FAST_HEX which improves
psnr by 0.56% without any noticeable speed drop (tested on gipsmotion).

Change-Id: Ic13176572dbd3aed5884a26786940a4b1bbd8a75