ALSA: hda - add DP mst verb support

Add snd_hda_get_dev_select() and snd_hda_set_dev_select() functions
for DP MST audio support.

Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Libin Yang <libin.yang@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1484208294-8637-2-git-send-email-libin.yang@intel.com