mesa: strengthen the condition that triggers generating VS with edge flags

Check the cull face state to see if polygon mode has any effect.
It could happen that polygon mode is GL_LINE, but that face is always
culled, in which case polygon mode has no effect, thus edge flags
have no effect either.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35314>
3 files changed