drm/meson: Add G12A Support for VIU setup

Amlogic G12A SoC needs a different VIU setup code,
handle it.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190325141824.21259-5-narmstrong@baylibre.com