PCI/PM: Call .bridge_d3() hook only if non-NULL

26ad34d5 ("PCI / ACPI: Whitelist D3 for more PCIe hotplug ports") added
the struct pci_platform_pm_ops.bridge_d3() function pointer and
platform_pci_bridge_d3() to use it.

The .bridge_d3() op is implemented by acpi_pci_platform_pm, but not by
mid_pci_platform_pm.  We don't expect platform_pci_bridge_d3() to be called
on Intel MID platforms, but nothing in the code itself would prevent that.

Check the .bridge_d3() pointer for NULL before calling it.

Fixes: 26ad34d5 ("PCI / ACPI: Whitelist D3 for more PCIe hotplug ports")
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>