drm/i915: Add atomic_get_property entrypoint for connectors (v2)

Even though we only support atomic plane updates at the moment, we still
need to add an .atomic_get_property() entrypoint for connectors before
we allow the driver to flip on the DRIVER_ATOMIC bit.  As soon as that
bit gets set, the DRM core will start adding atomic connector properties
(in addition to the plane properties we care about at the moment), so we
need to be able to handle the new way the DRM core will interact with
us.

For simplicity, we just lookup driver-specific connector properties in
the usual shadow array maintained by the core.  Once we get real atomic
modeset support for crtc's and planes, this code should be re-written to
pull the data out of crtc/connector state structures.

v2: Fix intel_dvo and intel_dsi that I missed on the first pass (Ander)

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>