dma-fence: remove duplicate word in comment

Signed-off-by: Frank Binns <frank.binns@imgtec.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1508333423-5394-1-git-send-email-frank.binns@imgtec.com