drm/atomic: Clean up update_output_state.

With the addition of crtc_state->connector_mask other connectors from
different crtc's aren't needed any more to determine if a crtc has
connectors, so only call add_affected_connectors on the target crtc.
This allows a cleanup to first remove all current connectors, then
add all set->connectors to the target crtc.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1456303053-28806-2-git-send-email-maarten.lankhorst@linux.intel.com