wayland/dma-buf: Prefer sync_file for dma-buf readiness source
Similarly to !3389 (merged), this avoids over-synchronization due to the client reading from the buffer, e.g. if drawing a frame re-uses the previous frame contents.
Similarly to !3389 (merged), this avoids over-synchronization due to the client reading from the buffer, e.g. if drawing a frame re-uses the previous frame contents.