perf hists: Do not link a pair if already linked

When we have multiple events in a group we link hist_entries in the
non-leader evsel hists to the one in the leader that points to the same
sorting criteria, in hists__match().

For 'perf report' we do this just once and then print the results, but
for 'perf top' we need to look if this was already done in the previous
refresh of the screen, so check for that and don't try to link again.

This is part of having 'perf top' using the hists browser for showing
multiple events in multiple columns.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-iwvb37rgb7upswhruwpcdnhw@git.kernel.org


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