gallium: Fix NIR validation when lowering polygon stipple

The fmul operation takes the maximum number of components from either
of its operands. We only need to use 2 components from the fragment
coordinates.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7507>
1 file changed