tools/power turbostat: Include umask=%x in perf counter's config

Some counters, like cpu/cache-misses/, expose and require umask=%x
parameter alongside event=%x in the sysfs perf counter's event file.
This change make sure we parse and use it when opening user added
counters.

Signed-off-by: Patryk Wlazlyn <patryk.wlazlyn@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>