drm/sun4i: mixer: Fix layer init code

drm/sun4i: mixer: Fix layer init code

Code refactoring dropped extra NULL sentinel entry at the end of the drm
planes array.

Add it back.

Reported-by: Chen-Yu Tsai <wens@kernel.org>
Closes: https://lore.kernel.org/linux-sunxi/CAGb2v65wY2pF6sR+0JgnpLa4ysvjght5hAKDa1RUyo=zEKXreg@mail.gmail.com/
Fixes: 4fa45b04a47d ("drm/sun4i: layer: move num of planes calc out of layer code")
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Reviewed-by: Chen-Yu Tsai <wens@kernel.org>
Link: https://patch.msgid.link/20260218183454.7881-1-jernej.skrabec@gmail.com
[wens@kernel.org: Fix "Fixes" commit hash]
Signed-off-by: Chen-Yu Tsai <wens@kernel.org>