drm: vc4-firmware-kms: Remove incorrect overscan support.

The overscan support was required for the old mailbox API
in order to match up the cursor and frame buffer planes.
With the newer API directly talking to dispmanx there is no
difference, therefore FKMS does not need to make any
adjustments.

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