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