intel_pstate: change some inconsistent debug information

Commit ce717613 (intel_pstate: Turn per cpu printk into pr_debug)
turned per cpu printk into pr_debug.  However, only half of the change
was done, introducing an inconsistency between entry and exit from
driver pstate control.  This patch changes the exit message to pr_debug
also.

The various messages are inconsistent with respect to any identifier
text that can be used to help isolate the desired information from a
huge log.  This patch makes a consistent identifier portion of the
string.

Amends: ce717613 (intel_pstate: Turn per cpu printk into pr_debug)
Signed-off-by: Doug Smythies <dsmythies@telus.net>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>