drm/vc4: Switch DSI to the panel-bridge layer, and support bridges.

The newer version of the RPi panel driver is going to be a combination
of a bridge and a panel, but we should also support panels without a
bridge, so the panel-bridge layer lets us do that cleanly.

v2: Drop "dev" argument.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170511183128.25085-2-eric@anholt.net