ACPI / processor: Remove redundant NULL check before kfree

kfree() on a NULL pointer is a no-op, so remove a redundant NULL
pointer check in map_mat_entry().

[rjw: Changelog]
Signed-off-by: Syam Sidhardhan <s.syam@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>