In non-rd pickmode (speed >= 5), avoid duplication of computations in
model_rd_for_sb_y when the speed feature use_simple_block_yrd is
enabled (or for high bitdepth build under certain conditions).

QVGA, VGA and HD have 1.23%, 2.68% and 1.7% speedup on ARM for speed 8,
respectively.

Encoding results are bitexact for speed >= 5.

Change-Id: I3f9130810c21439f5ad7e159e21cb2243dcd05f1