perf list: Remove blank lines, headers when piping output

perf list: Remove blank lines, headers when piping output

So that one can, for instance, use it with wc -l:

  # perf list *:*write* | wc -l
  60

Or to look for the "bio" tracepoints, without 'perf list' headers:

  # perf list *:*bio* | head
    block:block_bio_backmerge                          [Tracepoint event]
    block:block_bio_bounce                             [Tracepoint event]
    block:block_bio_complete                           [Tracepoint event]
    block:block_bio_frontmerge                         [Tracepoint event]
    block:block_bio_queue                              [Tracepoint event]
    block:block_bio_remap                              [Tracepoint event]
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-ts7sc0x8u4io4cifzkup4j44@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>