drm/vc4: Switch DPI to using the panel-bridge helper.

Another 100 lines of boilerplate gone, while allowing for bridges to
be connected in the display chain.

Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170511183128.25085-3-eric@anholt.net