Make pci_enable_ptm() accessible from the drivers.

Exposing this to the driver enables the driver to use the
'ptm_enabled' field of 'pci_dev' to check if PTM is enabled or not.

This reverts commit ac6c26da ("PCI: Make pci_enable_ptm() private").

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>