drm/ast: Fix connector leak during driver unload

When unloading the ast driver, a warning message is printed by
drm_mode_config_cleanup() because a reference is still held to one of
the drm_connector structs.

Correct this by calling drm_crtc_force_disable_all() in
ast_fbdev_destroy().

Signed-off-by: Sam Bobroff <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/ast/ast_fb.c

···
 {
 	struct ast_framebuffer *afb = &afbdev->afb;
 
+	drm_crtc_force_disable_all(dev);
 	drm_fb_helper_unregister_fbi(&afbdev->helper);
 
 	if (afb->obj) {