drm/meson: Add G12A support for OSD1 Plane

Amlogic G12A SoC supports now up to 3 OSD planes (1 more than the
previous SoCs) and a brand new OSD plane blender module.

This patch uses the same OSD1 plane for G12A, using the exact same scaler
and OSD1 setup registers, except using the new blender register to
disable the plane.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
[narmstrong: fixed 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/20190325141824.21259-6-narmstrong@baylibre.com