drm/i915: move fence lru to struct drm_i915_fence_reg

This lru tracks fences, not objects, so move it to where it belongs.
As a side effect, this nicely shrinks drm_i915_gem_object by two
pointers.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Eric Anholt <eric@anholt.net>