drm/i915: Introduce i915_gem_object_finish_gpu()

... reincarnated from i915_gem_object_flush_gpu(). The semantic
difference is that after calling finish_gpu() the object no longer
resides in any GPU domain, and so will cause the GPU caches to be
invalidated if it is ever used again.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>