i965: Fix computation of abs(-x) in FS

i965: Fix computation of abs(-x) in FS

When updating a register reference to reflect the fact that we were
taking its absolute value, the fragment shader back-end failed to
clear the negate flag, resulting in abs(-x) getting computed as
-abs(x).

I also found (and fixed) a similar problem in brw_eu.h, but I'm not
aware of an actual manifestation of that problem.

Fixes piglit test glsl-fs-abs-neg-with-intermediate.