PCI / PM: Allow runtime PM without callback functions

a9c8088c ("i2c: i801: Don't restore config registers on runtime PM")
nullified the runtime PM suspend/resume callback pointers while keeping the
runtime PM enabled.

This caused the SMBus PCI device to stay in D0 with
/sys/devices/.../power/runtime_status showing "error" when the runtime PM
framework attempted to autosuspend the device.  This is due to PCI bus
runtime PM, which checks for driver runtime PM callbacks and returns
-ENOSYS if they are not set.

Since i2c-i801.c doesn't need to do anything device-specific for runtime
PM, Jean Delvare proposed this be fixed in the PCI core rather than adding
dummy runtime PM callback functions in the PCI drivers.

Change pci_pm_runtime_suspend()/pci_pm_runtime_resume() so they allow
changing the PCI device power state during runtime PM transitions even if
the driver supplies no runtime PM callbacks.

This fixes the runtime PM regression on i2c-i801.c.

It is not obvious why the code previously required the runtime PM
callbacks.  The test has been there since the code was introduced by
6cbf8214 ("PCI PM: Run-time callbacks for PCI bus type").

On the other hand, a similar change was done to generic runtime PM
callbacks in 05aa55dd ("PM / Runtime: Lenient generic runtime pm
callbacks").

Fixes: a9c8088c ("i2c: i801: Don't restore config registers on runtime PM")
Reported-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: stable@vger.kernel.org	# v4.18+