overlays: dpi18 and dpi24 vc4 compatibility

The dpi overlays use the fb device tree node as a place to hang the
necessary pinctrl changes. With one of the VC4 overlays loaded, the
fb node is disabled so the changes have no effect.

Modify the overlays to also use the vc4 node, to cover both use
cases.

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