drm/sun4i: layer: support just backend formats when frontend is unavailable

In some cases, such as running a new kernel with an old device tree that
has the frontend disabled, the backend's matching frontend might be
unavailable.

When this happens, the layers should only declare support for formats
that the backend support. This partially reverts commit 1c29d263
("drm/sun4i: Rename sun4i_backend_layer_formats to sun4i_layer_formats")
by bringing back sun4i_backend_layer_formats, and passing it to
drm_universal_plane_init, while also dropping the modifiers list,
in the event no frontend is available.

Fixes: b636d3f9 ("drm/sun4i: frontend: Add support for the BGRX8888 input format")
Fixes: 9afe52d5 ("drm/sun4i: frontend: Add support for semi-planar YUV input formats")
Fixes: 8c8152bf ("drm/sun4i: frontend: Add support for planar YUV input formats")
Fixes: b2ddf277 ("drm/sun4i: layer: Add tiled modifier support and helper")
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/20190125032314.20915-6-wens@csie.org