For the chroma plane, the selected filter length was based on
transform size as follows:
TX_4X4 -> filter_length = 4
TX_8X8 -> filter_length = 8
TX_16X16 -> filter_length = 6 (this is intentionally *not* 16)

This seems inconsistent; presumably the intention is to use the
6-tap filter for TX_8X8 as well. This patch makes the appropriate
change to make that be the case.

Change-Id: I7f53d1dce4f16144bcf0c20131527b5193311603