Prashant -
It looks like you don't need to generate composite sync after all. Vsync and Hsync go directly to the VGA connector. But both the SYNC/ and BLANK/ inputs to the ADV7123 are driven by the FPGA so you have to do something with them. Read the ADV7123 data sheet. And you can download the FPGA source code from TerasIC if you have an account.
You have all the information you need to make this work, and you're close. Not much else I can do for you. Maybe this video will help:
https://www.youtube.com/watch?v=wk5ft5rd1su Bob