[mlir][sparse] Calling `SparseTensorType::get{Pos,Crd}Type` directly

The `SparseTensorType` versions of these methods have some special handling to ensure that they work for unannotated tensors; whereas, the `stt.getEncoding().get{Pos,Crd}Type()` idiom can cause segfaults.

Reviewed By: Peiming

Differential Revision: https://reviews.llvm.org/D150815