I know that Data refers to Geordi as "Lt. La Forge" in the episode "Hero Worship" (when talking to Timothy). This is in the third person, though. When speaking to each other, do Data and Geordi ever speak formally (addressing by rank, or Geordi calling Data "sir")?
No such dialogue, it seems
Restricting to canon, I cannot locate such formal dialogue between Data and Geordi. (I'll be happy for someone with a better recollection of any minute pieces of dialogue, or better Google-Fu, to find an example.)
As early as "The Naked Now" (Season 1, Episode 2), we hear Data refer to La Forge simply as "Geordi":
RIKER: All I have is a vague memory of reading somewhere about someone taking a shower in his or her clothing.
DATA: Ah. The body Geordi discovered.
Then in "Code of Honour" (Season 1, Episode 3), we already hear Data and Geordi conversing with one another in a relaxed, purely informal manner:
DATA: Why that razor, my friend? Why not the one I adjusted to perfect efficiency?
LA FORGE: Shaving is a human art form, Data. Technological perfection can shave too close.
Also, from the same scene:
DATA: How do you know when something is funny?
LA FORGE: It's not explainable. You just do.
DATA: Perhaps it is you, Geordi. Includling the kiddillies, I've learned six hundred sixty two jokes, and you have not —
LA FORGE: Includling the kiddillies! Now, see, that's funny.
It seems, now that I think about it, that something is missing from the early TNG episodes: just how did Data and La Forge become such best buds? Apparently such a scene was written but never actually filmed, according to Levar Burton:
...and this is little-known Trek lore — in the audition sides for the character, during the casting process, there was a scene that Gene Roddenberry wrote between Geordi and Data, that established their relationship. And in that scene, Geordi and Data discussed how Geordi's eyes and Data's brain saw the world in a very similar way. They saw the truth of the world around them. And so, in the scene that Gene wrote, they formed a team, and they called themselves the Perceivers, because their perceptions were alike.
And even though the Perceivers thing never made it to the pilot episode, the relationship between Geordi and Data certainly did.
Without this scene, we simply find Data and La Forge communicating on informal terms, left to wonder when they dropped the formalities.
Data directly addresses Geordi as 'Commander La Forge' on at least two occasions (although the first one is a fudge since he's contacting him on behalf of the Enterprise).
DATA: Enterprise to Commander La Forge. I am reading significant power fluctuations from the station core. Please report.
DATA: Commander La Forge, please report.
LAFORGE: We're fine, Data. The power grid is fully restored.
As far as Lt. Commander La Forge referring to Lt. Commander Data (the ship's Second Officer) as "Sir", it should be noted that neither holds a senior rank after La Forge's promotion to Lt. Commander less than a year into the show's run