Create named constants for the minimum TX_SIZE used in the luma
plane, as well as the minimum allowed TX_SIZE for which we have to
explicitly code a selection (i.e., one larger than the minimum luma
TX_SIZE).

Then, use these constants consistently with tx_size_to_depth() to
index the tx_size counts in FRAME_COUNTS, and also consistently use
TX_SIZE named constants to index the tx_size_implied counts.

Failing to index these counts correctly can, e.g., cause tx_mode to
be chosen incorrectly.

Change-Id: I706a62a33e2282e67c97a68bade87fb8023ec13b