perf streams: Get the evsel_streams by evsel_idx

In previous patch, we have created evsel_streams array.

This patch returns the specified evsel_streams according to the
evsel_idx.

Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20201009022845.13141-3-yao.jin@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>