This adds support for emulating the Silicon Image SII9022 DVI/HDMI
bridge. It's not very clever right now, it just acknowledges
the switch into DDC I2C mode and back. Combining this with the
existing DDC I2C emulation gives the right behavior on the Versatile
Express emulation passing through the QEMU EDID to the emulated
platform.

Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>
Message-id: 20180227104903.21353-5-linus.walleij@linaro.org
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
[PMM: explictly reset ddc_req/ddc_skip_finish/ddc]
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>