backend/drm: fix missing wlr_output_state.allow_artifacts

Without allow_artifacts, applying the new state will fail because
it requires ALLOW_MODESET.

Fixes VT switch and disabling CRTCs.

backend/drm/backend.c

@@ -115,6 +115,7 @@ static void handle_session_active(struct wl_listener *listener, void *data) {
 			}
 			struct wlr_output_state state = {
 				.committed = committed,
+				.allow_artifacts = true,
 				.enabled = mode != NULL,
 				.mode_type = WLR_OUTPUT_STATE_MODE_FIXED,
 				.mode = mode,

backend/drm/drm.c

@@ -1050,6 +1050,7 @@ static void dealloc_crtc(struct wlr_drm_connector *conn) {
 
 	struct wlr_output_state output_state = {
 		.committed = WLR_OUTPUT_STATE_ENABLED,
+		.allow_artifacts = true,
 		.enabled = false,
 	};
 	struct wlr_drm_connector_state conn_state = {0};