drm/bridge: Add RGB to VGA bridge support

Some boards have an entirely passive RGB to VGA bridge, based on DACs
implemented by resistor ladders.

Those might or might not have an i2c bus routed to the VGA connector in
order to access the screen EDIDs.

Add a bridge that doesn't do anything but expose the modes available on the
screen, either based on the EDIDs if available, or based on the XGA
standards.

Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20160930143709.1388-3-maxime.ripard@free-electrons.com