vp10: extend range for delta Q values.
See issue 1051. 6 bits is fairly arbitrary but at least allows writing delta Q values that are fairly normal in other codecs. I can extend to 8 if people want full range, although I personally don't have any need for that. Change-Id: I0a5a7c3d9b8eb3de4418430ab0e925d4a08cd7a0