perf hist: Simplify __hpp_fmt() using hpp_fmt_data

The struct hpp_fmt_data is to keep the values for each group members so
it doesn't need to check the event index in the group.

Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240607202918.2357459-3-namhyung@kernel.org