drm/i915: Only perform set-to-gtt domain for objects bound to the global gtt

If an object is not bound into the global GTT, then it cannot be
accessed via the GTT. This restores the original code that was muddled
by ppGTT. In the process, we remove a WARN that had long outlived its
usefulness and was simply being coded around instead.

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