drm/dp-mst: Remove tx_down_in_progress

Just replicates whether the list is empty or not. Nuke code
to avoid writing docs for it!

Cc: Dave Airlie <airlied@redhat.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1468612088-9721-6-git-send-email-daniel.vetter@ffwll.ch