drm/i915: Remove the double-list iteration from bound_any()

The purpose of the function is to find out whether the object is still
bound in any address space. This can be easily checked by looking at the
vma currently associated with the object, rather than asking if any of
the global address spaces have an active vma on the object.

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