drm/crtc: only store the necessary data for set_config rollback

drm_crtc_helper_set_config only potentially touches connector->encoder
and encoder->crtc, so we only have to store those for all connectors
and encoders, respectively.

Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>