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

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

Clear the old_state and new_state pointers for private objects
in drm_atomic_state_default_clear(). We don't actually have
functions to get the new/old state for private objects so
getting access to the potentially stale pointers requires a
bit more manual labour than for other object types. But let's
clear the pointers for private objects as well, if only to
avoid future surprises when someone decides to add the functions
to get at them.

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

Cc: <stable@vger.kernel.org> # v4.14+
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Abhay Kumar <abhay.kumar@intel.com>
Fixes: a4370c777406 (drm/atomic: Make private objs proper objects)
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>