We lose even config space access when we power gate the ISP
via the PUNIT. That makes lspci & co. produce gibberish.

To fix that let's try to implement actual runtime pm hooks
and inform the pci core that the device always goes to
D3cold. That will cause the pci core to resume the device
before attempting config space access.

This introduces another annoyance though. We get the
following error every time we try to resume the device:
intel_atomisp2_pm 0000:00:03.0: Refused to change power state, currently in D3

The reason being that the pci core tries to put the device
back into D0 via the standard PCI PM mechanism before
calling the driver resume hook. To fix this properly
we'd need to infiltrate the platform pm hooks (could
turn ugly real fast), or use pm domains (which don't
seem to exist on x86), or some extra early resume
hook for the driver (which doesn't exist either).
So maybe we just choose to live with the error?

Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Darren Hart <dvhart@infradead.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>