enable trellis quantization for 2nd order blocks

Experimented with different value for Y2_RD_MULT ranging f[1, 32],
without adapting the value to MB coding mode/frame type/Q value,
4 works out best among all values, providing overall 0.1% coding
gain on the test set.

Change-Id: I6b2583a8aa5db5e7e5c65c646301909c0c58f876