drm/meson: Add G12A Support for the Overlay video plane

Amlogic G12A SoC supports the same set of Video Planes, but now
are handled by the new OSD plane blender module.

This patch uses the same VD1 plane for G12A, using the exact same scaler
and VD1 setup registers, except using the new blender register to
disable the plane.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
[narmstrong: fix typo in commit log]
Tested-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190325141824.21259-7-narmstrong@baylibre.com