perf hists browser: Elide DSO column when it is set to just one DSO, ditto for threads

perf hists browser: Elide DSO column when it is set to just one DSO, ditto for threads

And also no leed to show the [.] (level: k, . for userspace) when
showing just one DSO.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-4h3f6ro5o7ebepjbssxf0dd3@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>