[mlir][linalg][bufferize][NFC] Simplify buffer API of BufferizationState

Instead of `lookupBuffer` and `getResultBuffer`, there is now a single `getBuffer` function. This simplifies the `BufferizableOpInterface` API and is less confusing to users. They could previously have called the wrong function.

Furthermore, since `getBuffer` now takes an `OpOperand &` instead of a `Value`, users can no longer accidentally use one of the previous two functions incorrectly, which would have resulted in missing buffer copies.

Differential Revision: https://reviews.llvm.org/D116455