Preheat oven to 375 F. In a large pot on medium heat, melt the butter. Add the onion, garlic and mushrooms, saute until onion is translucent. Add chicken and cook until no longer any pink is showing. Add the Cream of Mushroom, Cream of Chicken, Cheddar Cheese soup, Diced Tomatoes and Chili Powder. Stir until well blended. Stir in the cooked white rice and pour into a greased 9 x 12 casserole dish. Bake for 30 minutes or until the center is bubbling. Sprinkle on grated cheddar cheese and let set until cheese is melting. Serve hot.