[mlir][linalg] Use getIteratorTypeArray instead of raw iterator_type attribute.

Summary:
Also modify helper methods to take StringRefs instread of Attributes. It makes
the code cleaner and will help with future migration from StringRef to
utils::IteratorType ([RFC](https://discourse.llvm.org/t/rfc-enumattr-for-iterator-types-in-linalg/64535)).

Differential Revision: https://reviews.llvm.org/D134888