perf evsel: Add pmu_name helper

Add helper to get the name of the evsel's PMU. This handles the case
where there's no sysfs PMU via parse_events event_type helper.

Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: James Clark <james.clark@linaro.org>
Tested-by: Leo Yan <leo.yan@arm.com>
Tested-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20250109222109.567031-2-irogers@google.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>