This CL doesn't change any coding behavior.
The functions modified in this CL include
choose_smallest_tx_size()
choose_largest_tx_size()
choose_tx_size_type_from_rd()
txfm_rd_in_plane()
txfm_yrd()

This change make the code cleaner and will facilitate rd_debug
implementation

Change-Id: I19b1b2384dce8b46b28d4c9baf80ff92998d05b6