drm: Free atomic state during cleanup

The current state of CRTCs, planes and connectors currently leaks during
DRM driver ->unload() unless drivers explicitly clean it up. Since there
is nothing driver-specific about it, that cleanup can be done within the
DRM core.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>