drm/sun4i: add a Kconfig option for sun4i-backend

As sun4i-backend is now a dedicated module, add an Kconfig option for
it to make it optional, since some build may only use other engines.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
diff --git a/drivers/gpu/drm/sun4i/Makefile b/drivers/gpu/drm/sun4i/Makefile
index a251fb3..da561d0 100644
--- a/drivers/gpu/drm/sun4i/Makefile
+++ b/drivers/gpu/drm/sun4i/Makefile
@@ -9,6 +9,7 @@
 sun4i-backend-y += sun4i_backend.o sun4i_layer.o
 
 obj-$(CONFIG_DRM_SUN4I)		+= sun4i-drm.o sun4i-tcon.o
-obj-$(CONFIG_DRM_SUN4I)		+= sun4i-backend.o
 obj-$(CONFIG_DRM_SUN4I)		+= sun6i_drc.o
 obj-$(CONFIG_DRM_SUN4I)		+= sun4i_tv.o
+
+obj-$(CONFIG_DRM_SUN4I_BACKEND)		+= sun4i-backend.o