remove mutext unlock of c->iolock in _xcb_conn_wait

I encountered an issue with mesa log: Swapchain status changed to VK_ERROR_SURFACE_LOST_KHR when I tried to run 3dmark on wayland. This change is trying to fix it by removing mutex unlock of c->iolock in _xcb_conn_wait.

This MR is to discuss why c->iolock was unlocked in previous code and how can this impact the result of poll(c->fd, ...).