drm/meson: overlay: setup overlay for Amlogic FBC Memory Saving mode

Setup the Amlogic FBC decoder for the VD1 video overlay plane to use
a different superblock size for the Memory Saving mode.

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/20200703080728.25207-5-narmstrong@baylibre.com