Unbinding doesn't always lead to unconditional destruction
of vma. This destruction avoidance happens if vma is part of
execbuffer relocation list or if vma is being considered for
eviction in i915_gem_evict_something().

For those other users, mark the vma unbound so that
the correct state of this vma is preserved.
Reported-by: NChris Wilson <chris@chris-wilson.co.ok>
Cc: Chris Wilson <chris@chris-wilson.co.ok>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NMika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>