perf config: Refactor a duplicated code for obtaining config file name

We were doing the same sequence to figure out what is the config
pathname to use, fix it by doing it before those two uses.

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1493209268-5543-2-git-send-email-treeze.taeung@gmail.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>