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>