perf hists: Introduce hist_entry__init function

perf hists: Introduce hist_entry__init function

Move the 'struct hist_entry' initialization code to a separate function.
It'll be useful and more clear for the following patches that introduce
allocation callbacks.

Releasing the hist_entry object in hist_entry__new function
(where it's allocated) rather than in hist_entry__init.

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/1467701765-26194-2-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>