drm/atomic: Avoid connector to writeback_connector casts

Use container_of() instead of type casting so that it keeps working
even if base is moved inside the drm_writeback_connector struct.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180703075022.15138-2-boris.brezillon@bootlin.com