anv: fix descriptor pool leak in VMA object

Quoting the spec :

   "When a pool is destroyed, all descriptor sets allocated from the
    pool are implicitly freed and become invalid. Descriptor sets
    allocated from a given pool do not need to be freed before
    destroying that descriptor pool."

This implies we might leak nodes allocated in the vma object.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 0a6d2593 ("anv: Allocate descriptor buffers from the BO cache")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <!7796>
36 jobs for master in 23 seconds (queued for 2 seconds)
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