intel/compiler: Convert wm_prog_key::persample_interp to a tri-state

intel/compiler: Convert wm_prog_key::persample_interp to a tri-state

This allows for the possibility that we may not know at compile time if
sample shading is enabled through the API.  While we're here, also
document exactly what this bit means so we don't confuse ourselves.

v2: Fixup coarse pixel values (Lionel)

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21094>