Decompose choose_tx_size_from_rd into three functions that determine
the transform coding rd at different stages. Besides the original
function, txfm_yrd() calculates the rd for fixed size and type.
choose_tx_size_fix_type() fixes the type and searches for the size.
It can enable other experiments to do restricted tx searches so as to
reduce the impact on speed.
Similar refactoring is done for select_tx_type_yrd() in VAR_TX.

Performance change in baseline is trivial:
0.014/0.001/-0.020 for lowres/midres/hdres.

Change-Id: I2ecbf6066329be088ec1bfb69013b657b14b8afe