drm/sun4i: Move drm_mode_config_cleanup call to main driver

drm_mode_config_cleanup is the complement of drm_mode_config_init, which
is called in the bind function of sun4i_drv. drm_mode_config_cleanup
should be put in the unbind function to match.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>