perf hists: Use struct perf_hpp_fmt::idx in perf_hpp__reset_width

perf hists: Use struct perf_hpp_fmt::idx in perf_hpp__reset_width

We are going to add dynamic hpp format fields, so we need to make the
'len' change for the format itself, not in the perf_hpp__format
template.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1453109064-1026-5-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>