drm/atomic: Clean old_state/new_state in drm_atomic_state_default_clear()

drm/atomic: Clean old_state/new_state in drm_atomic_state_default_clear()

Clear the old_state and new_state pointers for every object in
drm_atomic_state_default_clear(). Otherwise
drm_atomic_get_{new,old}_*_state() will hand out stale pointers to
anyone who hasn't first confirmed that the object is in fact part of
the current atomic transcation, if they are called after we've done
the ww backoff dance while hanging on to the same drm_atomic_state.

For example, handle_conflicting_encoders() looks like it could hit
this since it iterates the full connector list and just calls
drm_atomic_get_new_connector_state() for each.

And I believe we have now witnessed this happening at least once in
i915 check_digital_port_conflicts(). Commit 8b69449d2663 ("drm/i915:
Remove last references to drm_atomic_get_existing* macros") changed
the safe drm_atomic_get_existing_connector_state() to the unsafe
drm_atomic_get_new_connector_state(), which opened the doors for
this particular bug there as well.

v2: Split private objs out to a separate patch (Daniel)

Cc: stable@vger.kernel.org
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Abhay Kumar <abhay.kumar@intel.com>
Fixes: 581e49fe6b41 ("drm/atomic: Add new iterators over all state, v3.")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180502183247.5746-1-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sean Paul <seanpaul@chromium.org>