msm: mdp: Skip overlay start from overlay on if handoff pending

If handoff is pending, we should skip the overlay start and kickoff
and return success for overlay on. This is to ensure that fb open
succeeds for all displays despite handoff being pending.

Change-Id: If75f3a4ea61c48ee6b3c97096d20c592723fa97c
Signed-off-by: Camus Wong <camusw@codeaurora.org>