How to do frame rate conversion?

If all you want to do is scale 1080p60 to 720p60, you should be able to do it with the following three VIP blocks:

CVI -> Scaler -> CVO

All three of these should use the same clock for their Avalon streaming interface. This clock would generally be faster than your fastest pixel clock, but you can get away with a little less as once you take into account the horizontal blanking, the average rate is actually less than the pixel clock.

The CVI also needs a clock for it's video input. This would be the 148.5 MHz clock from your DVI receiver.

The CVO needs a clock for it's video output. You can use a PLL to divide the 148.5 MHz input clock to 74.25 MHz for this purpose.

There is no frame rate issue to worry about. The scaler will just output data at half the rate of it's input. The scalers Avalon streaming output will just be idle more than half the time.