drm: Remove destroy listener from list when fired

Looks like we missed this one during the conversion to
weston_signal_emit_mutable.

Found by running weston under valgrind and running/killing
weston-simple-dmabuf-egl

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>

libweston/backend-drm/fb.c

@@ -512,6 +512,8 @@ drm_fb_handle_buffer_destroy(struct wl_listener *listener, void *data)
 	struct drm_buffer_fb *buf_fb =
 		container_of(listener, struct drm_buffer_fb, buffer_destroy_listener);
 
+	wl_list_remove(&buf_fb->buffer_destroy_listener.link);
+
 	if (buf_fb->fb) {
 		assert(buf_fb->fb->type == BUFFER_CLIENT ||
 		       buf_fb->fb->type == BUFFER_DMABUF);