drm/i915: Split audio component to a generic type

For allowing other drivers to use the DRM audio component, rename the
i915_audio_component_* with drm_audio_component_*, and split the
generic part into drm_audio_component.h.  The i915 specific stuff
remains in struct i915_audio_component, which contains
drm_audio_component as the base.

The license of drm_audio_component.h is kept to MIT as same as the the
original i915_component.h.

This is a preliminary change for further development, and no
functional changes by this patch itself, merely code-split and
renames.

v1->v2: Use SPDX for drm_audio_component.h, fix remaining i915
        argument in drm_audio_component.h

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>