This is the final step for more generic support of DRM audio
component.  The generic audio component code is now moved to its own
file, and the symbols are renamed from snd_hac_i915_* to
snd_hdac_acomp_*, respectively.  The generic code is enabled via the
new kconfig, CONFIG_SND_HDA_COMPONENT, while CONFIG_SND_HDA_I915 is
kept as the super-class.

Along with the split, three new callbacks are added to audio_ops:
pin2port is for providing the conversion between the pin number and
the widget id, and master_bind/master_unbin are called at binding /
unbinding the master component, respectively.  All these are optional,
but used in i915 implementation and also other later implementations.

A note about the new snd_hdac_acomp_init() function: there is a slight
difference between this and the old snd_hdac_i915_init().  The latter
(still) synchronizes with the master component binding, i.e. it
assures that the relevant DRM component gets bound when it returns, or
gives a negative error.  Meanwhile the new function doesn't
synchronize but just leaves as is.  It's the responsibility by the
caller's side to synchronize, or the caller may accept the
asynchronous binding on the fly.

v1->v2: Fix missing NULL check in master_bind/unbind

Signed-off-by: Takashi Iwai <tiwai@suse.de>