i965/gen6+: Add support for noperspective interpolation.

i965/gen6+: Add support for noperspective interpolation.

This required the following changes:

- WM setup now makes the appropriate set of barycentric coordinates
  (perspective vs. noperspective) available to the fragment shader,
  based on whether the shader requires perspective interpolation,
  noperspective interpolation, both, or neither.

- The fragment shader backend now uses the appropriate set of
  barycentric coordiantes when interpolating, based on the
  interpolation mode returned by
  ir_variable::determine_interpolation_mode().

- SF setup now uses gl_fragment_program::InterpQualifier to determine
  which attributes are to be flat shaded (as opposed to the old logic,
  which only flat shaded colors).

- CLIP setup now ensures that the clipper outputs non-perspective
  barycentric coordinates when they are needed by the fragment shader.

Fixes the remaining piglit tests of interpolation qualifiers that were
failing:
- interpolation-flat-*-smooth-none
- interpolation-flat-other-flat-none
- interpolation-noperspective-*
- interpolation-smooth-gl_*Color-flat-*

Reviewed-by: Eric Anholt <eric@anholt.net>