nir/i915g/r300/nv30: skip marking varyings as flat in some drivers

nir/i915g/r300/nv30: 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: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6500
Fixes: 7647023f3bb5 ("glsl: enable the use of the nir based varying linker")

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16573>