Whisk the chicken broth, 2 tablespoons chili powder, 1 teaspoon cumin, the tomatoes and poblano in a 6- to 8-quart slow cooker. Tear 1 tortilla into pieces and add to the slow cooker. Mix the remaining 1 tablespoon chili powder and 1/2 teaspoon cumin in a small bowl. Toss the cheese with 1/4 cup cilantro in a separate bowl. Set aside.
Butterfly the steak: Put the steak on a cutting board with the short end facing you, then slice in half horizontally, with your knife parallel to the board, without cutting all the way through; open like a book. (If your steak is less than 1/2 inch thick to begin with, do not butterfly; instead, lightly pound the steak with the flat side of a meat mallet until about 1/4 inch thick.) Season with salt, pepper and the chili powder-cumin mixture.
Press the cheese mixture over the steak in an even layer and arrange the remaining 2 whole tortillas in the center, overlapping slightly if needed. Starting with a long side, roll up the steak into a tight log. Tie with kitchen twine in 1 1/2-inch intervals, then tuck in the ends and tie lengthwise to keep the ends in place. Nestle the steak roll in the slow cooker and spoon some of the sauce over the top. Cover and cook on low until the meat is tender and the sauce is bubbling, 7 hours.
Remove the steak roll to a cutting board and let rest, 5 minutes. Stir the corn into the slow cooker, cover and continue to cook on low until heated through, about 5 minutes, then stir in the remaining 1/4 cup cilantro. Untie the steak and thinly slice. Serve with the sauce.