drm/i915: clean-up i915_gem_flush_gpu_write_domain

Now that we have an exact gpu write domain tracking, we don't need
to move objects to the active list ourself. i915_add_request will
take care of that under all circumstances.

Idea stolen from a patch by Chris Wilson <chris@chris-wilson.co.uk>.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>