PCI/MSI: Move power state check out of pci_msi_supported()

27e20603 ("PCI/MSI: Move D0 check into pci_msi_check_device()")
moved the power state check into pci_msi_check_device(), which was
subsequently renamed to pci_msi_supported().  This didn't change the
behavior, since both callers checked the power state.

However, it doesn't fit the current "pci_msi_supported()" name, which
should return what the device is capable of, independent of the power
state.

Move the power state check back into the callers for readability.  No
functional change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>