drm_hwcomposer: CI: Enable merge request pipelines

Now, freedesktop forbids running user pipelines to avoid hacker attacks.
This change will ensure that only the drm_hwcomposer project runs the
pipeline.

Change-Id: I87b58c87b41cfd31fb41c49c0faf399950b95022
Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>

.gitlab-ci.yml

@@ -1,5 +1,10 @@
 image: ubuntu:23.04
 
+workflow:
+  rules:
+    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
+    - if: $CI_PIPELINE_SOURCE == 'push'
+
 variables:
   DEBIAN_FRONTEND: noninteractive