i965/brw_reg: struct constructor now needs explicit negate and abs values.

i965/brw_reg: struct constructor now needs explicit negate and abs values.

We were assuming, when constructing a new brw_reg struct, that the
negate and abs register modifiers would not be present by default in
the new register.

Now, we force explicitly setting these values when constructing a new
register.

This will avoid problems like forgetting to properly set them when we
are using a previous register to generate this new register, as it was
happening in the dFdx and dFdy generation functions.

Fixes piglit test shaders/glsl-deriv-varyings

Cc: "10.4 10.3" <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=82991
Reviewed-by: Matt Turner <mattst88@gmail.com>