David Hockney is one of the world's most successful and influential artists of our time. At 11, he wanted to be an artist. At 87, he's still creating new works. Happiest when making 'joyful' art, and always innovating with his iPad, his art has sold for millions. Ahead of his biggest show ever opening in Paris, the Bradford-born star talks to Katie Razzall about growing old, a recent visit from the King, and his two big loves - smoking and painting.