Add mesa-iris repo

This allows us to uprev and build iris on its own schedule from the
mesa-iris and arc-mesa-iris ebuild.

BUG=b:141490430
TEST=repo sync, mesa-iris repo is present

Change-Id: Ieb41d62f5d11310c02ba2a1e0a4c8a92d0615da4
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/manifest/+/1799502Reviewed-by: Bernie Thompson <bhthompson@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
Commit-Queue: Kristian H. Kristensen <hoegsberg@chromium.org>
Tested-by: Kristian H. Kristensen <hoegsberg@chromium.org>

full.xml

@@ -350,6 +350,9 @@ Your sources have been sync'd successfully.
   <project path="src/third_party/mesa-freedreno"
            name="chromiumos/third_party/mesa"
            revision="refs/heads/chromeos-freedreno" />
+  <project path="src/third_party/mesa-iris"
+           name="chromiumos/third_party/mesa"
+           revision="refs/heads/chromeos-iris" />
   <project path="src/third_party/mesa-img"
            name="chromiumos/third_party/mesa"
            revision="refs/heads/mesa-img" />