agp/nvidia: Convert to generic power management

Convert agpgart-nvidia from legacy PCI power management to the generic
power management framework.

Previously agpgart-nvidia used legacy PCI power management, and
agp_nvidia_suspend() and agp_nvidia_resume() were responsible for both
device-specific things and generic PCI things:

  agp_nvidia_suspend
    pci_save_state                         <-- generic PCI
    pci_set_power_state(PCI_D3hot)         <-- generic PCI

  agp_nvidia_resume
    pci_set_power_state(PCI_D0)            <-- generic PCI
    pci_restore_state                      <-- generic PCI
    nvidia_configure                       <-- device-specific

Convert to generic power management where the PCI bus PM methods do the
generic PCI things, and the driver needs only the device-specific part,
i.e.,

  suspend_devices_and_enter
    dpm_suspend_start(PMSG_SUSPEND)
      pci_pm_suspend                       # PCI bus .suspend() method
        agp_nvidia_suspend                 <-- not needed at all; removed
    suspend_enter
      dpm_suspend_noirq(PMSG_SUSPEND)
        pci_pm_suspend_noirq               # PCI bus .suspend_noirq() method
          pci_save_state                   <-- generic PCI
          pci_prepare_to_sleep             <-- generic PCI
            pci_set_power_state
    ...
    dpm_resume_end(PMSG_RESUME)
      pci_pm_resume                        # PCI bus .resume() method
        pci_restore_standard_config
          pci_set_power_state(PCI_D0)      <-- generic PCI
          pci_restore_state                <-- generic PCI
        agp_nvidia_resume                  # driver->pm->resume
          nvidia_configure                 <-- device-specific

Based on 0aeddbd0 ("via-agp: convert to generic power management") by
Vaibhav Gupta <vaibhavgupta40@gmail.com>.

Link: https://lore.kernel.org/r/20221025203852.681822-6-helgaas@kernel.org


Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Dave Airlie <airlied@redhat.com>