dma-buf/sync_file: free fences array in num_fences is 1

When merging sync_files there is a case when we can end up with only one
fence in the merged sync_file: when all fences belong to the same
timeline.

So for this case a fence_array is not created instead we just assigned the
fence to sync_file->fence. Then we do not use the fences array anymore nor
does free it.

This patch frees the array.

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1474442419-6040-1-git-send-email-gustavo@padovan.org