For the grits: Bring 4 cups of salted water to a boil in the top pot of a double boiler set over high heat. Meanwhile, fill the bottom pot of the double boiler about halfway with water and bring to a boil over high heat, then reduce heat to medium. Gradually pour grits into the salted water, stirring constantly with a wooden spoon to prevent lumps, then fit pot onto the bottom pot of the double boiler. As the grits absorb the liquid and thicken, about 2 minutes, add 2 cups of the milk and cook, stirring frequently, until grits are tender and creamy, about 45 minutes. (Thin with some of the remaining milk if grits become too thick.) Cover pot and reduce heat to low while preparing shrimp.
For the shrimp: Fry bacon in a medium skillet over medium heat until browned and crisp, then transfer with a slotted spoon to paper towels to drain. Discard all but a thin film of bacon drippings from skillet.
Increase heat to medium-high and add shrimp, garlic, and three-quarters of the bacon to same skillet; saute, stirring often, until shrimp are just pink, about 3 minutes. Add wine, scrape any brown bits stuck to bottom of skillet with a wooden spoon, and cook until alcohol has evaporated and reduced slightly, about 2 minutes. Add cream and cook, stirring constantly, until sauce has thickened, about 2 minutes more. Divide grits among four bowls, then spoon shrimp and sauce over grits.