dt-bindings: display: sun4i: Fix D1 pipeline count

When adding the bindings for the D1 display engine, I missed the
condition for the number of pipelines. D1 has two mixers, so it
will have two pipeline references.

Fixes: ae5a5d26 ("dt-bindings: display: Add D1 display engine compatibles")
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20220702032921.22433-1-samuel@sholland.org