PM / Runtime: Combine runtime PM entry points

This patch (as1424) combines the various public entry points for the
runtime PM routines into three simple functions: one for idle, one for
suspend, and one for resume.  A new bitflag specifies whether or not
to increment or decrement the usage_count field.

The new entry points are named __pm_runtime_idle,
__pm_runtime_suspend, and __pm_runtime_resume, to reflect that they
are trampolines.  Simultaneously, the corresponding internal routines
are renamed to rpm_idle, rpm_suspend, and rpm_resume.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>