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>

src/intel/ci/gitlab-ci-inc.yml

@@ -216,7 +216,7 @@
     RUNNER_TAG: mesa-ci-x86-64-lava-asus-C433TA-AJ0005-rammus
     FDO_CI_CONCURRENT: 5
 
-# 7 boards
+# 6 boards
 .iris-kbl-test:
   extends:
     - .iris-test

src/intel/ci/gitlab-ci.yml

@@ -169,7 +169,7 @@ iris-kbl-piglit:
   extends:
     - .iris-kbl-test
     - .lava-piglit
-  parallel: 3
+  parallel: 2
   variables:
     HWCI_START_XORG: 1
     PIGLIT_NO_WINDOW: 0