where can I find tvp5150 / saa7121 driver code on DM642 ?

I do not believe a driver was ever written for the SAA7121 on the DM642, your best bet would be to take the existing SAA7105 driver code and modify it to work with the SAA7121, I imagine this would primarily consist of modifying the initialization code (i2c register writes) for the SAA7121, assuming you are working with the same video standard.

ok,

Then where can I get the driver code for tvp5150?

I notice that EVMDM642 version3 is using tvp5150, then I think  there should be a DDK update for it.

Can you tell me where can I get it? 

Currently the only way to get the software on your own would be by purchasing the EVM which includes the software on a CD. In your case you should submit a request through http://support.ti.com for the DM642 EVM DMDK software package, they should be able to get it to you. I am looking into a way to get the software posted publicly to avoid the need to request it from support.