perf intel-pt: Tidy Intel PT evsel lookup into separate function

Tidy the lookup of the Intel PT selected event (perf_evsel) into a separate
function.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Link: http://lkml.kernel.org/r/1495786658-18063-30-git-send-email-adrian.hunter@intel.com


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