perf top: Fix top.call-graph config option reading

When trying to add the "call-graph" variable for top into the
.perfconfig file, like:

      [top]
            call-graph = fp

I that perf_top_config() do not parse this variable.

Fix it by calling perf_default_config() when the top.call-graph variable
is set.

Signed-off-by: Yisheng Xie <xieyisheng1@huawei.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: b8cbb349

 ("perf config: Bring perf_default_config to the very beginning at main()")
Link: http://lkml.kernel.org/r/1520853957-36106-1-git-send-email-xieyisheng1@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>