drm/i915: Convert intel_dp_mst connector properties to atomic.

drm/i915: Convert intel_dp_mst connector properties to atomic.

MST doesn't support setting any properties, but it should still
use the atomic helper for setting properties.

Only path and tile properties are supported (read-only).
Those are immutable, and handled by drm core.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1491815239-10685-4-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>