drm/i915: Remove unused dp properties for dp-mst.

Those properties are not hooked up on MST and were ignored. Best not expose them at all.
Without this the next patch fails to start on X.org, because the DP-MST properties could
not be read.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/751b85a0-81cd-09e2-9e60-6d4ddbf1c6ac@linux.intel.com


Testcase: kms_properties
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>