ci: Extend meson-clang coverage by compiling all gallium drivers

Some compile-time regressions go unnoticed because not all gallium
drivers are compiled as part of the meson-clang job. Let's pass an
explicit list of drivers to compile instead of setting GALLIUM_DRIVERS
to "auto" to increase CI coverage.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6811>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 119d99d..2af0ee8 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -745,7 +745,7 @@
     DRI_LOADERS: >
       -D glvnd=true
     DRI_DRIVERS: "auto"
-    GALLIUM_DRIVERS: "auto"
+    GALLIUM_DRIVERS: "iris,nouveau,kmsro,r300,r600,freedreno,swr,swrast,svga,v3d,vc4,virgl,etnaviv,panfrost,lima,zink,radeonsi,tegra"
     VULKAN_DRIVERS: intel,amd,freedreno
     CC: "ccache clang-9"
     CXX: "ccache clang++-9"