perf stat: Don't use ctx for saved values lookup

We don't need to use ctx to look up events for saved values.  The
context is already part of the evsel pointer, which is the primary key.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20170831194036.30146-9-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>