Restore thread if there's no interrupt handler set

If dispatch is disabled and we don't do this, it'll just hang, never switching back to a thread that can re-enable dispatch.  Normally, it would do this after returning from the interrupt handler.

Fixes #5172.

May fix other games.  It's uncommon to have no interrupt handler installed for vblank, but it does happen.  It's also uncommon for games to disable dispatch, but that also happens.  Not sure how often they both happen, but very possibly at least other games by this same developer.

-[Unknown]