perf cgroup: Make evlist__find_cgroup() more compact

perf cgroup: Make evlist__find_cgroup() more compact

By taking advantage that __get() routines return the pointer to the
object for which a reference count is being get.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-xnvd07rdxliy04oi062samik@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>