drm/sun4i: Add end of list element for sun4i_layers_init's returned list

The number of defined planes in sun4i_layer is unknown to other parts
of the sun4i drm driver. Since the return value of sun4i_layers_init
is a list of layers, make it return 1 more empty layer as an end of
list guard value.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>