drm/meson: Add G12A support for plane handling in CRTC driver

This patch adds support for the new OSD+VD Plane blending module
in the CRTC code by adding the G12A code to manage the blending
module and setting the right OSD1 & VD1 plane registers.

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/20190325141824.21259-8-narmstrong@baylibre.com