perf top: Set display thread COMM to help with debugging

When we want to attach just to the thread that updates the display it
helps having its COMM stand out, so change it from the default "perf" to
"perf-top-UI".

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-5w0hmlk3zfvysxvpsh763k9w@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>