ci/iris: Drop parallel on kbl piglit to 2.

ci/iris: Drop parallel on kbl piglit to 2.

We have only 6 of these boards since one died in May, and 7 jobs allocated
to them.  So you ended up with a 5 minute delay on each pipeline with an
otherwise-idle farm while you waited for the first batch of jobs to
complete so you could get the last one started.  It turns out that piglit
was taking 3 minutes of runtime each, so we can just shard piglit 2 ways,
stay under runtime, and not over-allocate the farm

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25790>