perf x86/topdown: Make topdown metrics comparators be symmetric

perf x86/topdown: Make topdown metrics comparators be symmetric

The commit "3b5edc0421e2 (perf x86/topdown: Don't move topdown metric
 events in group)" modifies topdown metrics comparator to move topdown
metrics events which are not in same group with previous event. But it
just modifies the 2nd comparator and causes the comparators become
asymmetric.

Thus modify the 1st topdown metrics comparator and make the two
comparators be symmetric, and refine the comments as well.

Suggested-by: Ian Rogers <irogers@google.com>
Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20241011110207.1032235-1-dapeng1.mi@linux.intel.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>