Replacing raster_block with block in the encoder.

We only used "ib" to call get_scan() function, which in turn calls
get_tx_type_4x4() function. The latter one only needs block index if
bsize < BLOCK_8X8 -- under that condition raster_block == block.

Change-Id: I697306a0c3cf937acdd4f5e623d4367c5acc0b2f