radv/ci: make use of the new VanGogh runners

I just added 3 new Steam Decks at my home CI, bringing the total count
to 5. Let's make use of 2 of these to reduce execution time in Mesa CI
and leave the last one available for DXVK-CI.

Signed-off-by: Martin Roukala (né Peres) <martin.roukala@mupuf.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19929>

src/amd/ci/gitlab-ci.yml

@@ -350,7 +350,7 @@ vkcts-navi21-llvm-valve:
     RADV_DEBUG: llvm,checkir
 
 vkcts-vangogh-valve:
-  parallel: 2
+  parallel: 4
   extends:
     - .vkcts-test-valve
     - .vangogh-test-valve