i965/fs: Switch W and 1/W in Sandybridge interpolation setup.

i965/fs: Switch W and 1/W in Sandybridge interpolation setup.

Various documentation mentions that "W" is handed to the WM stage,
but further digging seems to indicate that they really mean 1/W.

The code here is still unclear, but changing this fixes piglit
test "fragcoord_w" on Sandybridge as well as a Khronos ES2 conformance
test.  I also tested 3DMarkMobile ES2.0's taiji and hoverjet demos, as
well as Nexuiz, just to be safe.

NOTE: This is a candidate for the 7.10 branch.