ALSA: hda: export struct hda_intel

This struct will be needed by the component code added in an upcoming
patch, so export it into a new hda_intel.h file. At the same time also
merge hda_i915.h into this new header, there is no reason to keep two
separate intel specific header file.

Suggested-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>