ACPI / CPPC: acquire pcc_lock only while accessing PCC subspace

We need to acquire pcc_lock only when we are accessing registers
that are in the PCC subspsace.

Signed-off-by: Prashanth Prakash <pprakash@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>