x86 idle: rename global pm_idle to static x86_idle

(pm_idle)() is being removed from linux/pm.h
because Linux does not have such a cross-architecture concept.

x86 uses an idle function pointer in its architecture
specific code as a backup to cpuidle.  So we re-name
x86 use of pm_idle to x86_idle, and make it static to x86.

Signed-off-by: Len Brown <len.brown@intel.com>
Cc: x86@kernel.org