perf trace: Only auto set call-graph to "dwarf" when syscalls are being traced

perf trace: Only auto set call-graph to "dwarf" when syscalls are being traced

When --min-stack or --max-stack is passwd but --no-syscalls is also in
effect, there is no point in automatically setting '--call-graph dwarf'.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-pq922i7h9wef0pho1dqpttvn@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>