fixed a wrong intialization value

The "update" variable was used as a flag in coef_prob update dry run
that tests if a frame should encodes update at all. The wrong init
value forced the update happening always. fixing this has a minor
improvement in low bit rate situation when 8x8 transform is allowed.

Change-Id: Icb498e8d6a62fd074dcbc2065b797cba9237cb51