drm: vc4: Add an overlay plane to vc4-firmware-kms

This uses a new API that is exposed via the mailbox service
to stick an element straight on the screen using DispmanX.

The primary and cursor planes have also been switched to using
the new plane API, and it supports layering based on the DRM
zpos parameter.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>