backend/drm: fetch current CRTC once on startup

Once we are DRM master, the CRTC cannot be changed behind our back
except during a VT switch.

After a VT switch, we try to restore whatever KMS state we had last
programmed. Reloading the current CRTC from KMS breaks this and
can result in a modeset without a FB.

Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3432

backend/drm/drm.c

@@ -1363,14 +1363,14 @@ void scan_drm_connectors(struct wlr_drm_backend *drm,
 			snprintf(wlr_conn->name, sizeof(wlr_conn->name),
 				"%s-%"PRIu32, conn_name, drm_conn->connector_type_id);
 
+			wlr_conn->crtc = connector_get_current_crtc(wlr_conn, drm_conn);
+
 			wl_list_insert(drm->outputs.prev, &wlr_conn->link);
 			wlr_log(WLR_INFO, "Found connector '%s'", wlr_conn->name);
 		} else {
 			seen[index] = true;
 		}
 
-		wlr_conn->crtc = connector_get_current_crtc(wlr_conn, drm_conn);
-
 		// This can only happen *after* hotplug, since we haven't read the
 		// connector properties yet
 		if (wlr_conn->props.link_status != 0) {