Improve vp9_idct4x4_1_add_sse2

Simple modification to reduce number of cycles in the
function.
Original function number of cycles: 973
Modified function number of cycles: 835
Improvment factor: 1.165

Tested with: park_joy_420_720p50.y4m

Change-Id: Ic5857272ea3aafe21d5ef9a69258d78c688f69bd