perf annotate: Stop using map->groups, use map_symbol->mg instead

These were the last uses of map->groups, next cset will nuke it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-n3g0foos7l7uxq9nar0zo0vj@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>