Using update_ct and update_ct2 functions for probability update.

Update logic for both mode and mvref was the same, so using MODE_COUNT_SAT,
MODE_MAX_UPDATE_FACTOR, update_ct, update_ct2 for both cases. Removing
function update_tx_ct because it was identical to update_mode_ct2.

Change-Id: Iff566be27dbd6cde4c2ec04e8d988f207046b8f0