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")
(cherry picked from commit cfe69d0a)