[mlir] Update accessors to prefixed form (NFC)

Follow up from flipping dialects to both, flip accessor used to prefixed
variant ahead to flipping from _Both to _Prefixed. This just flips to
the accessors introduced in the preceding change which are just prefixed
forms of the existing accessor changed from.

Mechanical change using helper script
https://github.com/jpienaar/llvm-project/blob/main/clang-tools-extra/clang-tidy/misc/AddGetterCheck.cpp and clang-format.