drm/meson: Add G12A support for CVBS Encoder

The Meson G12A SoCs uses the exact same CVBS encoder except a simple
CVBS DAC register offset and settings delta.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
[narmstrong: fixed subject typo]
Tested-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190325141824.21259-9-narmstrong@baylibre.com