ALSA: hda - Move mutex from hda_eld to per_pin in HDMI codec driver

Since the lock is used primarily in patch_hdmi.c, it's better to move
it in the local struct instead of exporting in hda_eld.  The only
functions requiring the lock in hda_eld.c are proc accessors.  So in
this patch, the proc entry and its creation/deletion/accessors are
moved into patch_hdmi.c, together with the mutex lock to pin_spec
struct.

The former proc info functions are exported so that they can be called
from patch_hdmi.c.

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