perf stat: Skip evlist__[enable|disable] when all events uses BPF

When all events of a perf-stat session use BPF, it is not necessary to
call evlist__enable() and evlist__disable(). Skip them when
all_counters_use_bpf is true.

Signed-off-by: Song Liu <song@kernel.org>
Reported-by: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>