ALSA: hda - Implement independent HP control

Similar like the implementation in patch_analog.c and patch_via.c,
the generic parser can provide the independent HP PCM stream now.
It's enabled when spec->indep_hp is set by the caller while parsing.

Currently no dynamic PCM switching as in patch_via.c is implemented
yet.  The control returns -EBUSY when the value is changed during PCM
operations.

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