drm/vgem: Fix non static symbol warning

Fixes the following sparse warning:

drivers/gpu/drm/vgem/vgem_fence.c:75:24: warning:
  symbol 'vgem_fence_ops' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1468932262-26554-1-git-send-email-weiyj_lk@163.com