There's no mention of spirits gaining power by having worshippers. The only thing in the games that talk about "spirit power levels" are the codex entries about demons, written by Circle mages and enchanters. They say that spirits that reflect complex emotions are more powerful, and contrast Rage (weak) and Pride (strong). The only other way we've seen spirits gain power are the Baroness feeding on her subjects to become Pride, and the Nightmare (a spirit which reflects Fear of the Blight) feeding on people by taking their memories. That's less "being worshipped" and more "being a vampire".
Cole says how worship made "them" [evanuris<- tied to spirits] "more" and that Solas only wanted to spread knowledge. A fear demon feeds on fear, and a spirit of wisdom feeds on faith. Not really complicated. Solas is described as a rebel god, but his main purpose was spreading knowledge (wisdom) about the true nature of the Evanuris. It seems like his whole life before the Veil was dedicated to that. If you worship the knowledge of a spirit, you make it stronger. Ez.