backend-drm: Skip testing plane state if plane is not enabled

Adds a further assert() to make sure we're not checking against invalid
values. This was seen in the wild when the kernel rejects the commit for
overlay resulting in a check for invalid zpos values.

Fixes: #304

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

libweston/backend-drm/state-propose.c

@@ -604,6 +604,12 @@ drm_output_check_zpos_plane_states(struct drm_output_state *state)
 		struct wl_list *next_node = ps->link.next;
 		bool found_dup = false;
 
+		/* skip any plane that is not enabled */
+		if (!ps->fb)
+			continue;
+
+		assert(ps->zpos != DRM_PLANE_ZPOS_INVALID_PLANE);
+
 		/* find another plane with the same zpos value */
 		if (next_node == &state->plane_list)
 			break;