nvc0: do upload-time fixups for interpolation parameters

nvc0: do upload-time fixups for interpolation parameters

Unfortunately flatshading is an all-or-nothing proposition on nvc0,
while GL 3.0 calls for the ability to selectively specify explicit
interpolation parameters on gl_Color/gl_SecondaryColor which would
override the flatshading setting. This allows us to fix up the
interpolation settings after shader generation based on rasterizer
settings.

While we're at it, we can add support for dynamically forcing all
(non-flat) shader inputs to be interpolated per-sample, which allows
st/mesa to not generate variants for these.

Fixes the remaining failing glsl-1.30/execution/interpolation piglits.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>