Fix multiple design uploads

What does this MR do?

This MR fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/12780, which is a bug where multiple design uploads were returning just a single result.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

/cc @phikai @dsatcher