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

Previously agpgart-ati used legacy PCI power management, and
agp_ati_suspend() and agp_ati_resume() were responsible for both
device-specific things and generic PCI things like saving and restoring
config space and managing power state:

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

  agp_ati_resume
    pci_set_power_state(PCI_D0)            <-- generic PCI
    pci_restore_state                      <-- generic PCI
    ati_configure                          <-- device-specific

With generic power management, 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_ati_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_ati_resume                     # driver->pm->resume
          ati_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/20221025203852.681822-5-helgaas@kernel.orgSigned-off-by: NBjorn Helgaas <bhelgaas@google.com>
Acked-by: NDave Airlie <airlied@redhat.com>