Adding update_tx_counts function.

Moving common encoder/decoder code to update_tx_counts. Also renaming
vp9_get_pred_probs_tx_size to get_tx_probs2 and adding get_tx_probs to
call vp9_get_pred_context_tx_size inside read_selected_tx_size only once
(twice before).

Change-Id: Ia50247f3893de88ef8e9041b0d44be44a40aaa4d