radv: Interpolate less aggressively.

Seems like dxvk used integer builtins without setting the flat interpolation decoration.

I believe in the current spec the app is required to set these, but in the meantime to avoid breaking things in stable releases (and so close to release for 19.0), only expand the interpolation to float16 and struct (which cannot be builtins as our spirv parser lowers the builtin block).

Fixes: f3247841 "radv: Allow interpolation on non-float types."