No, it’s not necessary at all to bring a laptop for studies at IIT KGP. While it is true that most professors use PPTs and PDFs to teach, you can always access them on your mobile phone. Some subjects might require programs like PDS, and you can go to the digital library and use computers.
A laptop might help you a bit but not having one will definitely not affect your scores or academic performance. In fact, a laptop might even become a source of distraction and might involve you in gaming.
So rest assured and enjoy IIT KGP campus life rather than spending your time in front of the laptop screen.