docs/ci: pass -D platforms=[] to avoid dependencies

Since commit 8f6fca89aa ("meson: delete dri3 build option"),
the docs build attempts to find dependencies like xshmfence which
are not found in the current docs image.
Avoid checking for those dependencies by not enabling any platforms,
which will skip the check overall.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31025>

docs/gitlab-ci.yml

@@ -8,7 +8,7 @@
       - public/
   script:
     - meson setup _build -D prefix=$(pwd) --auto-features=disabled
-      -D vulkan-drivers="" -D gallium-drivers="" -D glx=disabled
+      -D vulkan-drivers="" -D gallium-drivers="" -D platforms=[] -D glx=disabled
       -D video-codecs="" -D html-docs=enabled -D html-docs-path=public
     - meson compile -C _build
     - meson install -C _build