overlays: vc4-kms-v3d: Change composite handling

On a Pi 4, enabling composite video disables the HDMI output. As a
consequence, the composite output is disabled by default. Change the
vc4-kms-v3d overlay used on older Pis to also disable composite by
default, replacing the "nocomposite" parameter with a "composite"
parameter.

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