mesa/st: add per sample shading state to fp key and set interpolation

This enables a gallium driver not to care about the semantics of
ARB_sample_shading vs ARB_gpu_shader5 sample attributes. When
ARB_sample_shading-style sample shading is enabled, all of the fp inputs
are marked for interpolation at the sample location.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nv50,nvc0: remove hacky global per-sample interpolation setting

Now that inputs are correctly annotated with interpolation location,
don't try to divine whether all inputs should be per-sample-interpolated
or not.

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