drm/i915: Eliminate obj->state usage from pre/post plane update

Dig up the appropriate new crtc and plane states from the top level
atomic state in intel_pre_plane_update() and intel_post_plane_update().

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170823152226.22938-5-ville.syrjala@linux.intel.com