ci: don't build whole Xorg in per-driver jobs

The per-driver jobs only need the SDK headers, so we can speed things up
and save a lot CPU time by not actually building the whole Xorg source.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

.gitlab-ci.yml

@@ -276,7 +276,7 @@ xf86-driver-build-test:
     variables:
         GIT_DEPTH: 1
         MESON_ARGS: -Dprefix=/usr/
-        MESON_EXTRA_ARGS: -Dxwayland=false -Dxnest=false -Dxvfb=false -Dxquartz=false -Ddocs=false
+        MESON_EXTRA_ARGS: -Dxwayland=false -Dxnest=false -Dxvfb=false -Dxquartz=false -Ddocs=false -Dxorg_sdk=true -Dxorg=false
     rules:
       - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
         changes: