Moving from ss_txfrm_size to tx_size.

Updating foreach_transformed_block_visitor and corresponding functions
to accept tx_size instead of ss_txfrm_size. List of functions per file:

vp9_decodframe.c
  decode_block
  decode_block_intra

vp9_detokenize.c
  decode_block

vp9_encodemb.c
  optimize_block
  vp9_xform_quant
  vp9_encode_block_intra

vp9_rdopt.c
  dist_block
  rate_block
  block_yrd_txfm

vp9_tokenize.c
  set_entropy_context_b
  tokenize_b
  is_skippable

Change-Id: I351bf563eb36cf34db71c3f06b9bbc9a61b55b73