drm/i915: Fix unsafe loop iteration over vma whilst unbinding them

On non-LLC platforms, when changing the cache level of an object, we may
need to unbind it so that prefetching across page boundaries does not
cross into a different memory domain. This requires us to unbind
conflicting vma, but we did so iterating over the objects vma in an
unsafe manner (as the list was being modified as we iterated).

The regression was introduced in
commit 3089c6f2
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Wed Jul 31 17:00:03 2013 -0700

    drm/i915: make caching operate on all address spaces
apparently as far back as v3.12-rc1, but it has only just begun to
trigger real world bug reports.

Reported-and-tested-by: Nikolay Martynov <mar.kolya@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76384


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>