perf evlist: Add toggle_enable() method

For an upcoming feature in 'perf top' we will have a hotkey to
enable/disable events, so remember if the events in the list are
enabled or disabled and allows toggling this state using a new
method.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-64c4jvdl5feg2zhimxvokqka@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>