perf python: Use attr.watermark in twatch.py

We want to be woken up for every PERF_RECORD_ event, attr.wakeup_events
is only for PERF_RECORD_SAMPLE, so also use attr.watermark = 1 to fix
that.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
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-v3lnpwgrr8mllcr3ntduuqvc@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>