drm/sun4i: Allow VI layers to be primary planes

D1's mixer 1 has no UI layers, only a single VI layer. That means the
mixer can only be used if the primary plane comes from this VI layer.
Add the code to handle this case.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220424162633.12369-10-samuel@sholland.org