Hindsight's 20:20, mate.
In Flotsam, Geralt has no idea where these ghosts may be or when he might find them, even assuming he believed the 'prophecy' which is by no means certain. Triss, on the other hand has the guaranteed cure for his amnesia and the knowledge of how to apply it. So what's more likely? That he goes chasing off after some hairy fairy prophecy he lacks a location for or goes after the person with who he knows without doubt can cure him? Triss is key for that reason. The fact that he found other means and to cure his amnesia prior to rescuing/finding her is by the by.
It's not just hindsight, maybe it's a reason to progress on to chapter 2 but during that chapter he makes it clear that he knows already that clearing the mist is the key to returning his memory, and it does, all before we finally get to her at the end of chapter 3 So while going after her because she could be the key to returning his memory is valid reason to progress to chapter two, once you're there it is clearly not the case anymore and there's different reason to continue to go after her.