Corrected value range of --cpu-used for vp9

This commit removes undefined value options of cpu-used for VP9 and
changed vpxenc prompt to reflect the usable range of [-8,8]

Change-Id: Ib80fef3dbb6ec9aabac45ed13e8ab6fbaf94f55e
Merge "Corrected value range of --cpu-used for vp9"