i965/fs: use determine_interpolation_mode().

i965/fs: use determine_interpolation_mode().

This patch changes how fs_visitor::emit_general_interpolation()
decides what kind of interpolation to do.  Previously, it used the
shade model to determine how to interpolate colors, and used smooth
interpolation on everything else.  Now it uses
ir_variable::determine_interpolation_mode(), so that it respects GLSL
1.30 interpolation qualifiers.

Fixes piglit tests interpolation-flat-*-smooth-{distance,fixed,vertex}
and interpolation-flat-other-flat-{distance,fixed,vertex}.

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