perf pmu: Change convert_scale from static to global

perf pmu: Change convert_scale from static to global

The function convert_scale() can be used to convert string to unit and
scale. For example,

  s = "6000000000ns";
  convert_scale(s, &unit, &scale);

unit = "ns", scale = 6000000000.

Currently this function is static. This patch renames the function to
perf_pmu__convert_scale and changes the function to global.  No
functional change.

Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20190828055932.8269-2-yao.jin@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>