v3d-rpi4-gl: reduce the parallelism from 10 to 8

We are slightly over-subscribed right now, with 21 merge jobs (10 vk
+ 10 gl + 1 traces) for 20 RPi4, so let's split the tests between
slightly fewer RPi4 and make each split job run for slightly longer,
because it also means that all the jobs start immediately, reducing the
overall delay in merging any MR that triggers this job.

The typical run time for this job was around 8 min with a 10-split; with
an 8-split it is now 9 min, which is still within the 10 min target and
well below the 15min limit.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27391>

src/broadcom/ci/gitlab-ci.yml

@@ -27,7 +27,7 @@ v3d-rpi4-gl:arm64:
     - .igalia-bcm2711-rpi-4:arm64
     - .broadcom-test:arm64
     - .v3d-rules
-  parallel: 10
+  parallel: 8
   variables:
     HWCI_START_WESTON: 1
     DEQP_SUITE: broadcom-rpi4