Make Masala filling:
Peel potatoes and cut into 1 1/2-inch pieces. Transfer to a bowl and cover with cold water.
Toast coconut in a 12-inch heavy skillet over medium heat, stirring occasionally, until golden, about 3 minutes. Transfer to a small bowl and wipe out skillet. Toast cumin seeds in skillet over medium heat, shaking skillet frequently, until fragrant and just a shade darker, about 30 seconds. Transfer to another small bowl. Reserve skillet.
Purée jalapeño, ginger, and garlic in a blender with curry powder, cinnamon, turmeric, oil, 1/4 cup water, and 1 teaspoon salt until smooth. Transfer purée to skillet and cook over medium-high heat, stirring, until thickened slightly, about 1 minute. Add onion and cook, stirring occasionally, until it begins to soften, about 8 minutes.
Drain potatoes, then add to onion mixture with cumin seeds and cook over medium heat, stirring occasionally, until potatoes are barely tender, about 10 minutes.
Add chickpeas and remaining 1 1/2 cups water, scraping up any brown bits, then briskly simmer, covered, until potatoes are tender, 16 to 20 minutes more. Add peas and cook, covered, until just tender, about 3 minutes. Remove from heat and stir in toasted coconut and cilantro.
Make dosas while potatoes cook:
Whisk flours, cumin seeds, salt, and water in a bowl.
Generously brush a 12-inch nonstick skillet with oil and heat over medium-high heat until it shimmers. Pour 1/2 cup batter into skillet, swirling until bottom is coated. Cook, undisturbed, until dosa is set and edges are golden, about 2 minutes. Flip using a rubber spatula and cook dosa until underside is golden in spots, about 1 minute more. Transfer to a plate. Make more dosas with remaining batter, stacking and covering loosely with foil to keep warm. To serve, spoon masala filling into dosas.