drm/atomic: atomic plane properties

Expose the core plane state as properties, so they can be updated via
atomic ioctl.

v2: atomic property flag

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>