drm/i915: Update atomic state when removing mst connector, v3.

Fully remove the MST connector from the atomic state, and remove the
early returns in check_*_state for MST connectors.

With atomic the state can be made consistent all the time.

Thanks to Sivakumar Thulasimani for the idea of using
drm_atomic_helper_set_config.

Changes since v1:
- Remove the MST check in intel_connector_check_state too.
Changes since v2:
- Use drm_atomic_helper_set_config.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>