drm/dp/mst: save vcpi with payloads

This makes it possibly for drivers to find the associated
mst_port by looking at the payload allocation table.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449514552-10236-3-git-send-email-harry.wentland@amd.com


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>