When cutting power to a GPU and its integrated HDA controller, their
cached current_state should be updated to D3cold to reflect reality.

We currently rely on the DRM and HDA drivers to do that, however:

- The HDA driver updates the current_state in azx_vs_set_state(), which
  will no longer be called with driver power control once we migrate to
  device links.  (It will still be called with manual power control.)

- If the HDA device is not bound, its current_state remains at D0 even
  though the GPU driver may decide to go to D3cold.

- The DRM drivers update the current_state using pci_set_power_state()
  which can't put the device into a deeper power state than D3hot if the
  GPU is not deemed power-manageable by the platform (even though it
  *is* power-manageable by some nonstandard means, such as a _DSM).

Centralize updating the current_state of the GPU and HDA controller in
vga_switcheroo's ->runtime_suspend hook to overcome these deficiencies.

The GPU and HDA controller are two functions of the same PCI device
(VGA class device on function 0 and audio device on function 1) and
no other PCI devices reside on the same bus since this is a PCIe
point-to-point link, so we can just walk the bus and update the
current_state of all devices.

On ->runtime_resume, the HDA controller is in D0uninitialized state.
Resume to D0active and then let it autosuspend as it sees fit.

Note that vga_switcheroo_init_domain_pm_ops() is not supposed to be
called by hybrid graphics laptops which power down the GPU via its root
port's _PR3 resources and consequently vga_switcheroo_runtime_suspend()
is not used.  On those laptops, the root port is power-manageable by the
platform (instead of by a nonstandard means) and the current_state is
therefore updated by the PCI core through the following call chain:

  pci_set_power_state()
    __pci_complete_power_transition()
      pci_bus_set_current_state()

Resuming to D0active happens through:

  pci_set_power_state()
    __pci_start_power_transition()
      pci_wakeup_bus()

Cc: Dave Airlie <airlied@redhat.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Tested-by: Kai Heng Feng <kai.heng.feng@canonical.com> # AMD PowerXpress
Tested-by: Mike Lothian <mike@fireburn.co.uk>          # AMD PowerXpress
Tested-by: Denis Lisov <dennis.lissov@gmail.com>       # Nvidia Optimus
Tested-by: Peter Wu <peter@lekensteyn.nl>              # Nvidia Optimus
Tested-by: Lukas Wunner <lukas@wunner.de>              # MacBook Pro
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: https://patchwork.freedesktop.org/patch/msgid/8416958482c8c42d6f311ea5c1e5a65ccf21f5db.1520068884.git.lukas@wunner.de