According to Wikipedia's article on photosynthetic efficiency, sugarcane, which is abnormally efficient, can store 8% of the energy of sunlight it receives. Let's say that your genetic engineering is pretty good, so the pigs are able to reach this level of efficiency.
Sunlight is 120 W/m2, which is equivalent to 0.029 Calories per second, or about 104 Calories per hour of sunlight. Let's say there are 10 hours worth of sunlight during the day, both for ease of calculation and to account for dawn and dusk not having as much light. That leaves us with a potential of about 1000 Calories per day of sunlight.
Given that pigs can grow to almost 2 meters long, and that they're longer than they are wide, let's just say the pigs will have 1 m2 of exposed skin.
Putting these together, your pig will get about 80 Calories per day from sunlight. I found this source that says an adult pig should get 9000 Calories per day (I think this is for pigs that are being raised for meat). That's less than 1% of their daily caloric needs.
Based on these numbers, I don't expect this to happen any time in the near future.
It would take a lot of time and effort in order to figure out how to get the 8% efficiency of sugarcane instead of the more typical 1% efficiency of most crops. Then, once you've reached that point, you get a very marginal benefit.