"Warlord" and "landlord" are English words. 

"Lord of war" and "lord of land" do not happen to be established English phrases, but they can certainly be used: but they do not mean the same as the words. (For example, a "landlord" may be quite a humble person, provided only that they have some property which is let to somebody else - and that is leaving aside the special meaning of "landlord" as "publican" or "proprietor of a pub"). 

In a feudal setting, you could reasonably use the phrase "lord of the land" - this would mean the lord who ruled over the land: a quite different meaning from "landlord". 

In the case the Rings, neither "Ringlord" nor "Lord of the rings" existed in English before Tolkien wrote them, as far as I know. We cannot know why he used the phrase for the title, but my guess is 


it has a more sonorous sound
at the time he wrote it, I think that using a neologism like "Ringlord" in the title of a book would have puzzled both editors and readers. 


when to use "of" and when not?
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