drm/i915: Copy the staged connector config to the legacy atomic state

With this in place, we can start converting pieces of the modeset code
to look at the connector atomic state instead of the staged config.

v2: Handle the load detect staged config changes too. (Ander)
    Remove unnecessary blank line. (Daniel)

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>