drm/sun4i: Fix kcalloc element size in sun4i_layers_init

In sun4i_layers_init we are allocating an array of pointers to struct
sun4i_layer:

	layers = devm_kcalloc(drm->dev, ARRAY_SIZE(sun4i_backend_planes),
	                      sizeof(**layers), GFP_KERNEL);

The element size should be the size of an individual element of the
array. Change it to sizeof(*layers) to avoid wasting a lot of memory.

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