drm/i915: Convert fence computations to use vma directly

Lookup the GGTT vma once for the object assigned to the fence, and then
derive everything from that vma.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471254551-25805-13-git-send-email-chris@chris-wilson.co.uk