drm/sun4i: Save newly created layer in layers array in sun4i_layers_init

sun4i_layers_init allocates an array to store pointers to newly created
layers returned by sun4i_layer_init_one(), but fails to actually store
them. But it actually returns the empty array to unsuspecting users.

Save the pointers in the array, so that they may be used later.

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