drm/vgem: Remember to offset relative timeouts to mod_timer() by jiffies

mod_timer() takes an absolute jiffie value, not a relative timeout and
quietly fixup the missed ret=0 otherwise gcc just always returns that
the fence timed out.

Testcase: igt/vgem_basic/fence
Fixes: 40777984 ("drm/vgem: Attach sw fences to exported vGEM dma-buf")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1468834278-26716-1-git-send-email-chris@chris-wilson.co.uk