PM / core: Simplify initcall_debug_report() timing

initcall_debug_report() always called ktime_get(), even if we didn't
need the result.

Change it so we only call it when we're going to use the result, and
change initcall_debug_start() to follow the same style.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>