ALSA - hda: hdmi check NULL pointer in hdmi_set_chmap

Make sure per_pin is not NULL before using it.

Fixes: 9b3dc8aa ('ALSA: hda - Register chmap obj as priv data instead of codec')
Signed-off-by: Libin Yang <libin.yang@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>