vc4: Fix vc4_fence_server_sync() on pre-syncobj kernels.
We won't have an FD if we're just having the server wait on a fence created by eglCreateSyncKHR(). Our seqno fences will happen in order, so server-side waits are no-ops in that case. Fixes dEQP-EGL.functional.sharing.gles2.multithread.simple_egl_server_sync.buffers.gen_delete Fixes: b0acc3a5 ("broadcom/vc4: Native fence fd support")