Whisk garlic, yogurt, 2 tablespoons oil, and 1/2 teaspoon sumac in a medium bowl; season with salt. Spoon sauce onto a platter. Place eggplants on top. Season with salt, drizzle with oil, and sprinkle with sumac. DO AHEAD: Yogurt and Sumac Sauce can be made 1 day ahead. Cover and chill.