drm/ast: Use managed mode-config init

Using drmm_mode_config_init() sets up managed release of modesetting
resources. The existing modesetting's finalizer is empty, so remove it.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702115029.5281-14-tzimmermann@suse.de