Convert amd64-agp from legacy PCI power management to the generic power
management framework.

Previously, amd64-agp used legacy PCI power management, and
agp_amd64_suspend() and agp_amd64_resume() were responsible for both
device-specific things and generic PCI things:

  agp_amd64_suspend
    pci_save_state(pdev)                <-- generic PCI
    pci_set_power_state(pdev, pci_choose_state(pdev, state)) <-- generic PCI

  agp_amd64_resume
    pci_set_power_state(pdev, PCI_D0)   <-- generic PCI
    pci_restore_state(pdev)             <-- generic PCI
    nforce3_agp_init()                  <-- device-specific
    amd_8151_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_amd64_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_amd64_resume                # dev->driver->pm->resume
          nforce3_agp_init()            <-- device-specific
          amd_8151_configure()          <-- device-specific

[bhelgaas: commit log]
Link: https://lore.kernel.org/r/20211208193305.147072-2-helgaas@kernel.orgSigned-off-by: Vaibhav Gupta <vaibhavgupta40@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Dave Airlie <airlied@redhat.com>