selftests/resctrl: Remove nested calls in perf event handling

Perf event handling has functions that are the sole caller of another
perf event handling related function:
  - reset_enable_llc_perf() calls perf_event_open_llc_miss()
  - perf_event_measure() calls get_llc_perf()

Remove the extra layer of calls to make the code easier to follow by
moving the code into the calling function.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>