drm/i915/gtt: Neuter the deferred unbind callback from gen6_ppgtt_cleanup

Having deferred the vma destruction to a worker where we can acquire the
struct_mutex, we have to avoid chasing back into the now destroyed
ppgtt. The pd_vma is special in having a custom unbind function to scan
for unused pages despite the VMA itself being notionally part of the
GGTT. As such, we need to disable that callback to avoid a
use-after-free.

This unfortunately blew up so early during boot that CI declared the
machine unreachable as opposed to being the major failure it was. Oops.

Fixes: d3622099c76f ("drm/i915/gtt: Always acquire struct_mutex for gen6_ppgtt_cleanup")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Tomi Sarvela <[email protected]>
Reviewed-by: Tvrtko Ursulin <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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