i915g: Fix backface stencil when front_ccw is set.

i915g: Fix backface stencil when front_ccw is set.

If we're doing separate front/back stencil ops, then in the absence of a
hardware bit for setting front_ccw (which you can already see from how we
manage cull face) we need to flip front/back stencil register settings.

Closes: #4974, #4975
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11706>