cpupower : Fix cpupower working when cpu0 is offline

cpuidle_monitor used to assume that cpu0 is always online which is not
a valid assumption on POWER machines. This patch fixes this by getting
the cpu on which the current thread is running, instead of always using
cpu0 for monitoring which may not be online.

Signed-off-by: Abhishek Goel <huntbag@linux.vnet.ibm.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>