drm/i915: Convert intel_dp properties to atomic, v2.

intel_dp supports 3 properties, scaling mode, broadcast rgb and
force_audio. intel_digital_connector handles the plumbing,
so we only have to hook this up in compute_config and init.

Changes since v1:
- Remove limited_color_range too, unused. (danvet)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170501133804.8116-10-maarten.lankhorst@linux.intel.com