drm/vc4: Attach margin props to the HDMI connector

Now that the plane code takes the margins setup into account, we can
safely attach margin props to the HDMI connector.

We also take care of filling AVI infoframes correctly to expose the
top/botton/left/right bar.

Note that those margin props match pretty well the
overscan_{left,right,top,bottom} properties defined in config.txt and
parsed by the VC4 firmware.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181206142439.10441-6-boris.brezillon@bootlin.com