drm: add support to sst-only sink branch device

Currently SST-only branch device is handled as MST branch device,
because SST-only branch device can't respond to MST LINK_ADDRESS
sideband message, it will fail the connector creation to any port
connected to the branch. This change treats SST-only sink branch
device as SST sink device and avoid LINK_ADDRESS failure.

Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>
Change-Id: I993e583c01f574ea428c1f5cec0f41eb169351dd