tools/power/turbostat: Fix a knl bug

MSR_KNL_CORE_C6_RESIDENCY should be evaluated only if
1. this is KNL platform
AND
2. need to get C6 residency or need to calculate C1 residency

Fix the broken logic introduced by commit 1e9042b9 ("tools/power
turbostat: Fix CPU%C1 display value").

Fixes: 1e9042b9

 ("tools/power turbostat: Fix CPU%C1 display value")
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>