When building with --enable-daala_ec, the tx_type for intra blocks can be
 coded using the CDFs that are updated once per frame.
This patch converts a tx_type symbol to be coded with aom_write_symbol()
 and aom_read_symbol() that was missed in f3e8e267.

Change-Id: I34f8fef7525f88e156bbcb78dfc48994367610ce