MIX cream cheese spread, milk and garlic powder until blended. Cook spaghetti in large saucepan as directed on package, omitting salt.
MEANWHILE, melt butter in large deep skillet on medium heat. Add chicken and pepper; cook and stir 4 min. or just until chicken is no longer pink. Add mushrooms; cook and stir 5 min. Add cream cheese mixture and peas; cook and stir 5 min. or until heated through.
DRAIN spaghetti. Add to chicken mixture in skillet with 3 Tbsp. Parmesan; toss to coat. Top with remaining Parmesan.