overlays: vc4-kms/fkms-v3d: Raise CMA to 512MB

On Pi 4, raise the requested CMA size to 512MB by default, on the
understanding that the firmware will cap that to 256MB if it's a
32-bit kernel with less than 2GB of RAM.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>