tools/power turbostat: add early exits for permission checks

Checking early if the permissions are even needed gets rid of the
warnings about some of them missing. Earlier we issued a warning in case
of missing MSR and/or perf permissions, even when user never asked for
counters that require those.

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