[mlir] Remove locking for dialect/operation registration.

Moving forward dialects should only be registered in a thread safe context. This matches the existing usage we have today, but it allows for removing quite a bit of expensive locking from the context.

This led to ~.5 a second compile time improvement when running one conversion pass on a very large .mlir file(hundreds of thousands of operations).

Differential Revision: https://reviews.llvm.org/D82595