PCI: PM: Rearrange pci_target_state()

Make pci_target_state() return D3cold or D0 without checking PME
support if the current power state of the device is D3cold or if it
does not support the standard PCI PM, respectively.

Next, drop the tergat_state local variable that has become redundant
from it.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Ferry Toth <fntoth@gmail.com>