x86: Enhance Cx stability by adding softirq check before entry cstate

x86: Enhance Cx stability by adding softirq check before entry cstate

Without checking softirq_pending before entry Cx state, softirq such
as SCHEDULE_SOFTIRQ and TIMER_SOFTIRQ can't be handled timely. It may
cause severe failures such as disk I/O failure.

This patch addresses the issue above, meanwhile move timing points
closer to cstate entry/exit point to make C3 residency more accurate,
and enable irq a little earlier.

Signed-off-by: Wei Gang <gang.wei@intel.com>
Signed-off-by: Tian Kevin <kevin.tian@intel.com>