drm/sun4i: Rename sun4i_backend_layer_formats to sun4i_layer_formats

Since more formats can be supported by the frontend, rename the
variable listing the layer formats to avoid suggesting that the backend
itself supports all the listed formats.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181123092515.2511-16-paul.kocialkowski@bootlin.com