perf annotate: Introduce annotation_line__max_percent()

Out of the annotate_browser__write() routine, to be used in the --stdio2
mode.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-0he0wyy4haswqi1qb35x37do@git.kernel.org


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