drm/i915: Setup dummy atomic state for connectors (v3)

We want to enable/test plane updates via the atomic interface, but as
soon as we flip DRIVER_ATOMIC on, the DRM core will take some atomic
codepaths to lookup properties during drmModeGetConnector() and some of
those codepaths unconditionally dereference connector->state
(specifically when looking up the CRTC ID property in
drm_atomic_connector_get_property()).  Create a dummy connector state
for each connector at init time to ensure the DRM core doesn't try to
dereference a NULL connector->state.  The actual connector properties
will never be updated or contain useful information, but since we're
doing this specifically for testing/debug of the plane operations (and
only when a specific kernel module option is given), that shouldn't
really matter.

Once we start creating connector states, the DRM core will want to be
able to clean them up for us.  We also need to hook up the destruction
entrypoint to the core's helper.

v2: Squash in the patch to set the state destruction hook (Ander & Bob)

v3: Only create dummy connector states when we're actually faking
    atomic support.  (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>