libvpx

Initialize best_tx_size with invalid value

If vp9_pick_inter_mode works properly, it should at least check
one coding mode and hence get best_tx_size assigned a valid value.
There is no need to initialize best_tx_size with a legitimate
value before starting the mode search.

Change-Id: Ic0496cd89672ea9c2c512a9bd1da952190af9cba