drm/sun4i: Add support for D1 mixers

D1 has a display engine with the usual pair of mixers, albeit with
relatively few layers. In fact, D1 appears to be the first SoC to have
a mixer without any UI layers. Add support for these new variants.

Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220424162633.12369-12-samuel@sholland.org