When Víkingur Ólafsson was 8 years old, he threw a tantrum over Wolfgang Amadeus Mozart. As he tried to play the runs in the deceptive little Sonata facile in C major, the so-called “Easy Sonata,” he grew so frustrated that he literally scratched out the notes with a pencil. Since then, the 37-year-old Icelandic pianist has made peace with Mozart. …