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>

drivers/gpu/drm/sun4i/Kconfig

@@ -12,3 +12,13 @@ config DRM_SUN4I
 	  Choose this option if you have an Allwinner SoC with a
 	  Display Engine. If M is selected the module will be called
 	  sun4i-drm.
+
+config DRM_SUN4I_BACKEND
+	tristate "Support for Allwinner A10 Display Engine Backend"
+	depends on DRM_SUN4I
+	default DRM_SUN4I
+	help
+	  Choose this option if you have an Allwinner SoC with the
+	  original Allwinner Display Engine, which has a backend to
+	  do some alpha blending and feed graphics to TCON. If M is
+	  selected the module will be called sun4i-backend.

drivers/gpu/drm/sun4i/Makefile

@@ -9,6 +9,7 @@ sun4i-tcon-y += sun4i_crtc.o
 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