drm/i915: Update dummy connector atomic state with current config

Keep that state updated so that we can write code that depends on it on
the follow up patches.

v2: Fix BUG due to stale connector_state->crtc value. (Chandra)

v3: Update comment about dummy state connectors. (Chandra)
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>