ALSA: hda - Enable runtime PM only for discrete GPU

The recent change of vga_switcheroo allowed the runtime PM for
HD-audio on AMD GPUs, but this also resulted in a regression.  When
the HD-audio controller driver gets runtime-suspended, HD-audio link
is turned off, and the hotplug notification is ignored.  This leads to
the inconsistent audio state (the connection isn't notified and ELD is
ignored).

The best fix would be to implement the proper ELD notification via the
audio component, but it's still not ready.  As a quick workaround,
this patch adds the check of runtime_idle and allows the runtime
suspend only when the vga_switcheroo is bound with discrete GPU.
That is, a system with a single GPU and APU would be again without
runtime PM to keep the HD-audio link for the hotplug notification and
ELD read out.

Also, the codec->auto_runtime_pm flag is set only for the discrete GPU
at the time GPU gets bound via vga_switcheroo (i.e. only dGPU is
forcibly runtime-PM enabled), so that APU can still get the ELD
notification.

For identifying which GPU is bound, a new vga_switcheroo client
callback, gpu_bound, is implemented.  The vga_switcheroo simply calls
this when GPU is bound, and tells whether it's dGPU or APU.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=200945
Fixes: 07f4f97d

 ("vga_switcheroo: Use device link for HDA controller")
Reported-by: Jian-Hong Pan <jian-hong@endlessm.com>
Tested-by: Jian-Hong Pan <jian-hong@endlessm.com>
Acked-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>