perf cs-etm: Use struct perf_cpu as much as possible

The perf_cpu struct makes some iterators simpler and avoids some
mistakes with interchanging CPU IDs with indexes etc. At the moment in
this file the conversion to an integer is done somewhere in the middle
of the call tree. Change it to delay the conversion to an int until the
leaf functions.

Some of the usage patterns are duplicated, so instead of changing them
all, make cs_etm_get_ro() more reusable and use that everywhere.
cs_etm_get_ro() didn't return an error before, but return one now so
that it can also be used where an error is needed. Continue to ignore
the error where it was already ignored.

Use cs_etm_pmu_path_exists() instead of cs_etm_get_ro() in
cs_etm_is_etmv4() because cs_etm_get_ro() prints a warning, but path
exists is sufficient for this use case.
Signed-off-by: James Clark <james.clark@arm.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: John Garry <john.g.garry@oracle.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Leo Yan <leo.yan@linux.dev>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mike Leach <mike.leach@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Suzuki Poulouse <suzuki.poulose@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20240501135753.508022-2-james.clark@arm.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>