ALSA: hda - Fix unused function in patch_intelhdmi.c

Add a proper ifdef to shut out a compile warning:
   CC [M]  sound/pci/hda/patch_intelhdmi.o
sound/pci/hda/patch_intelhdmi.c:286: warning: ‘hdmi_get_dip_index’ defined but \
not used

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