drm/atomic: Add __drm_atomic_helper_connector_reset, v2.

This is useful for drivers that subclass connector_state, like tegra.

Changes since v1:
- Docbook updates.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1451908400-25147-2-git-send-email-maarten.lankhorst@linux.intel.com


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>