drm/meson: overlay: setup overlay for Amlogic FBC

Setup the Amlogic FBC decoder for the VD1 video overlay plane.

The VD1 Amlogic FBC decoder is integrated in the pipeline like the
YUV pixel reading/formatter but used a direct memory address instead.

This adds support for the basic layout, and needs to calculate the content
body size since the header is allocated after.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200703080728.25207-4-narmstrong@baylibre.com