drm/vc4: Kick out the simplefb framebuffer before we set up KMS.

If we don't, then simplefb stays loaded on /dev/fb0 even though
scanout isn't happening from simplefb's memory area any more, and you
end up with no console.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Dave Airlie <airlied@redhat.com>