drm/vc4: Allow scaling on cursor plane

Now that async update has been reworked to allow scaled planes to be
updated asynchronously when the scaling params do not change, we can
remove the NO_SCALING constraint on cursor planes.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181130090254.594-5-boris.brezillon@bootlin.com