drm/vc4: Force trigger of dlist update on margins change

When the margins are changed, the dlist needs to be regenerated
with the changed updated dest regions for each of the planes.

Setting the zpos_changed flag is sufficient to trigger that
without doing a full modeset, therefore set it should the
margins be changed.

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