tools/power turbostat: Check for non-zero value when MSR probing

For some MSRs, for example, the Platform Energy Counter (RAPL PSYS), it
is required to additionally check for a non-zero value to confirm that
it is present.

From Intel SDM vol. 4:

    Platform Energy Counter (R/O)
    This MSR is valid only if both platform vendor hardware
    implementation and BIOS enablement support it.
    This MSR will read 0 if not valid.

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