drm/i915: Append the object onto the inactive list on binding.

In order to properly track bound objects, they need to exist on one of
the inactive/active lists or be pinned. As this is a requirement, do the
work inside i915_gem_bind_to_gtt() rather than dotted around the
callsites.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>