drm/i915: DocBook add i915_component.h support

Add the item of i915_component.h in DocBook and add the DOC for
i915_component.h. Explain the struct i915_audio_component_ops and
struct i915_audio_component_audio_ops usage.
Signed-off-by: Libin Yang <libin.yang@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Documentation/DocBook/drm.tmpl

@@ -4051,6 +4051,7 @@ int num_ioctls;</synopsis>
 	<title>High Definition Audio</title>
 !Pdrivers/gpu/drm/i915/intel_audio.c High Definition Audio over HDMI and Display Port
 !Idrivers/gpu/drm/i915/intel_audio.c
+!Iinclude/drm/i915_component.h
       </sect2>
       <sect2>
 	<title>Panel Self Refresh PSR (PSR/SRD)</title>

drivers/gpu/drm/i915/intel_audio.c

@@ -50,6 +50,11 @@
  * co-operation between the graphics and audio drivers is handled via audio
  * related registers. (The notable exception is the power management, not
  * covered here.)
+ *
+ * The struct i915_audio_component is used to interact between the graphics
+ * and audio drivers. The struct i915_audio_component_ops *ops in it is
+ * defined in graphics driver and called in audio driver. The
+ * struct i915_audio_component_audio_ops *audio_ops is called from i915 driver.
  */
 
 static const struct {