nir/i915g/r300: skip marking varyings as flat in some drivers
Some older drivers don't support GLSL versions with the concept of flat varyings and also don't support integers. Here we add a new setting to make sure we don't use the optimisation that sets varyings to flat. This setting helps us avoid marking varyings as flat and therefore potentially having them changed to ints via varying packing.
Closes: #6500 (closed) Fixes: 7647023f ("glsl: enable the use of the nir based varying linker")