drm/vc4: Drop planes that have 0 destination size

There is no point in trying to create a dlist entry for planes
that have a 0 crtc size, and it can also cause grief in the vc6
dlist generation as it takes width-1 and height-1, causing wrap
around.
Drop these planes.

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