dp/mst: add SDP stream support

This adds code to initialise the SDP streams
for a sink in the simplest ordering.

I've no idea how you'd want to control the
ordering at this level, so don't bother
until someone comes up with a use case.

Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Libin Yang <libin.yang@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449036584-105393-1-git-send-email-libin.yang@linux.intel.com