drm/sun4i: layer: Assign backend pointer before calling DRM helpers

We might want to use the backend pointer from DRM callbacks that get
called within drm_universal_plane_init(), such as the
.format_mod_supported callback.

Move the assignment of the layer's backend pointer to right after the
structure is allocated.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190125032314.20915-5-wens@csie.org