backend/drm: add wlr_drm_connector_state.nonblock

Instead of having this condition checked in multiple places,
centralize it so that they don't go out-of-sync.

include/backend/drm/drm.h

@@ -126,6 +126,7 @@ struct wlr_drm_mode {
 struct wlr_drm_connector_state {
 	const struct wlr_output_state *base;
 	bool modeset;
+	bool nonblock;
 	bool active;
 	drmModeModeInfo mode;
 	struct wlr_drm_fb *primary_fb;