foreach_transformed_block_in_plane cleanup, explicit tx_size var.

Making foreach_transformed_block_in_plane more clear (it's not finished
yet). Using explicit tx_size variable consistently instead of
(ss_txfrm_size / 2) or (ss_txfrm_size >> 1) expression.

Change-Id: I1b9bba2c0a9f817fca72c88324bbe6004766fb7d