perf tools: Add evlist__disable_evsel/evlist__enable_evsel

Adding interface to enable/disable single event in the evlist based on
its name. It will be used later in new control enable/disable interface.

Keeping the evlist::enabled true when one or more events are enabled so
the toggle can work properly and toggle evlist to disabled state.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Alexei Budankov <abudankov@huawei.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lore.kernel.org/r/20201210204330.233864-2-jolsa@kernel.org


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