Retries.for_inner_plan returns self when already in deferred

Summary:
If we are already in a defered retries state when for_inner_plan is requested - just remain in the current deferred mode instead of erroring out.

resolves #2805

Test Plan: not sure

Reviewers: sandyryza

Reviewed By: sandyryza

Differential Revision: https://dagster.phacility.com/D5124