drm/bridge: Clarify the atomic enable/disable hooks semantics

The [pre_]enable/[post_]disable hooks are passed the old atomic state.
Update the doc and rename the arguments to make it clear.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191203141515.3597631-8-boris.brezillon@collabora.com