AMD/ATI and Nvidia HDMI codec drivers didn't have the audio component
binding like i915, but it worked only with the traditional HD-audio
unsolicited event for the HDMI hotplug detection and the ELD read-up
thereafter.  This has been a problem in many ways: first of all, it
goes through the hardware event transition (from GPU register write,
HD-audio controller trigger, and finally to HD-audio unsolicited event
handling), which is often unreliable and may miss some opportunities.
Second, each unsol event handling and ELD read-up need the explicit
power up / down when the codec is in the runtime suspend.  Last but
not least, which is the most important, the hotplug wakeup may be
missed when the HD-audio controller is in runtime suspend.  Especially
the last point is a big problem due to the recent change relevant with
vga_switcheroo that forcibly enables the runtime PM for AMD HDMI
controllers.

These issues are solved by introducing the audio component; the
hotplug notification is done by a direct function callback, which is
more accurate and reliable, and it can be processed without the actual
hardware access, i.e. no runtime PM trigger is needed, and the
HD-audio gets the event even if it's in runtime suspend.  The same for
ELD query, as it's read directly from the cached ELD bytes stored in
the DRM driver, hence the whole hardware access can be skipped.

So here it is: this patch implements the audio component binding with
AMD/ATI and Nouveau DRM drivers.  The biggest difference from i915
implementation is that this binding is fully optional and it can be
enabled asynchronously on the fly.  That is, the driver will switch
from the HD-audio unsolicited event to the notify callback once when
the DRM component gets bound.  Similarly, when DRM driver gets
unloaded, the HDMI event handling returns to the legacy mode, too.

Also, another difference from i915 is that the new code registers the
component in the codec driver, while i915 HDMI codec assumes the
component binding was already done in the HD-audio controller driver.
Hence the new code does need to de-register the component binding at
the codec exit, too.

Some other details:
- The match component ops assumes that both VGA and HD-audio
  controller PCI entries belong to the same PCI bus, and only accepts
  such an entry.

- The pin2port audio_ops is implemented with assumption of the fixed
  widget layout.  For AMD, it's starting from 3, with step 2 (3, 5, 7,
  ...), while for Nvidia, it's starting from 4, with step 1 (4, 5, 6,
  ...)

As of this patch, the corresponding component isn't implemented in DRM
side, so this change alone won't give any benefit.  By the following
changes in DRM sides, the mission will be completed.
Signed-off-by: Takashi Iwai <tiwai@suse.de>