Use block_idx rather than block_raster_idx

1) block_raster_idx is actually raster order only when tx_size
is TX_4x8.
It's very specific, so we should put it near to the place it's
actually used.

2) Sync the meaning of block_idx on encoder/decoder sides

Change-Id: I7d37a992cb773503e29f9c0d9d2586e580aa6173
8 files changed