i965: Handle default interpolation modes and locations in NIR.

i965: Handle default interpolation modes and locations in NIR.

This consolidates a bunch of hacks in a single place - by setting
the interpolation modes and locations on variables appropriately,
we can simply trust them in the rest of the code.  This avoids
having to handle INTERP_QUALIFIER_NONE, gl_Color overrides,
sample-shading overrides, and Gen4-5 centroid-overrides in a bunch
of places.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>