drm/fsl-dcu: use drm_mode_config_cleanup on initialization errors

Commit 7566e247 ("drm/fsl-dcu: handle initialization errors properly")
introduced error handling during initialization, but with a wrong cleanup
order.

Replace the error handling with the generic cleanup function
drm_mode_config_cleanup.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160619021543.23587-1-stefan@agner.ch