perf hists browser: Fixup the "cpu" column width calculation

perf hists browser: Fixup the "cpu" column width calculation

Since we were not setting it to at least 3 chars ('CPU'), it was being
reset to zero when recalculating the columns width when refreshing the
screen, in 'perf top'. Fix it.

Tested-by: Wang Nan <wangnan0@huawei.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-iqcdnkkqm6sew06x01fbijmy@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>