I think it's meant to be "Which helmet is mine?" and you can only tell from the context. Compare with a phrase from Animals - "Sparior qintir ñuhor qintir issa?", translated as "What turtle is my turtle?". "Whose turtle is my turtle" would be incorrect (unless you were speaking to your turtle and tickling it under the chin as you spoke).
I think turtle is being used here as a "typical" noun for learning purposes. There is a phrase (in Commands) "Qintir emās, taobus" which means "Have a turtle, boy". And (in Basics 2) "Drogo qībōños zōbrie issa" - "Drogon's cloak is black" when Drogon is not really noted for wearing cloaks.