ALSA: x86: Fold intel_hdmi_audio_if.c into main file

As the very last step, we fold intel_hdmi_audio_if.c into the main
file, intel_hdmi_audio.c.  This is merely a cleanup, and no functional
change.

By this move, we can mark all functions and variables as static, which
allows the compiler more optimizations.

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