In Musing 1453, I wrote about the film Sarfarosh (1999) and surmised that the word Pakistani was used in this film for the first time. To which Rammesh Venugopal replied that it was not so, because the word Pakistan was used already in Border (1997).
So I watched the film Border (1997), based on the 1971 Indo-Pak War, and sure enough, the words East Pakistan and Pakistan were uttered in the first 15 minutes. So, until further research, this is the earliest film.
Of course, Rammesh would know, having written a very well-researched book on fifty Indian war movies. I also read his chapter on Border (1997) where he critiques the film and even finds about a dozen goofs.
Rammesh book on Amazon
Now You Know!