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

Previously, via-agp used legacy PCI power management, and agp_via_suspend()
and agp_via_resume() were responsible for both device-specific things and
generic PCI things:

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

  agp_via_resume
    pci_set_power_state(pdev, PCI_D0)   <-- generic PCI
    pci_restore_state(pdev)             <-- generic PCI
    via_configure_agp3()                <-- device-specific
    via_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_via_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_via_resume                  # dev->driver->pm->resume
          via_configure_agp3()          <-- device-specific
          via_configure()               <-- device-specific

[bhelgaas: commit log]
Link: https://lore.kernel.org/r/20211208193305.147072-4-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>