drm/dp/mst: reply with ACK for UP reqs

Currently we reply with NACK to UP requests which might
confuse receivers. We haven't seen any actual issues with
this but should still respond to UP requests correctly.

Signed-off-by: Mykola Lysenko <Mykola.Lysenko@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449514552-10236-2-git-send-email-harry.wentland@amd.com


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