[SYCL][NFC] Pass adapter by ref in ur::getAdapter and event:getAdapter (#19202)
It's a part of larger refactoring effort to pass adapter via reference
instead of pointer everywhere in the codebase.
Follow-up of:
https://github.com/intel/llvm/pull/19186
https://github.com/intel/llvm/pull/19184
https://github.com/intel/llvm/pull/19187