pm-graph: make setVal unbuffered again for python2 and python3

sleepgraph:
 - kprobe_events won't set correctly if the data is buffered
 - force sysvals.setVal to be unbuffered and use binary mode
 - tested in both python2 and python3

Link: https://bugzilla.kernel.org/show_bug.cgi?id=204773


Signed-off-by: Todd Brandt <todd.e.brandt@linux.intel.com>
[ rjw: Subject ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>