nir: Fix lower_two_sided_color when the face is an input

Fixes the two-sided-lighting and vertex-program-two-side piglit tests
on Panfrost.

v2: Use an existing variable for gl_FrontFacing if present.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Rob Clark <robdclark@chromium.org>
Tested-by: Urja Rannikko <urjaman@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5915>
1 file changed