anv/descriptor_set: Unlink sets from the pool in set_destroy

anv/descriptor_set: Unlink sets from the pool in set_destroy

anv_descriptor_pool_free_set is called on the clean-up path of
anv_descriptor_set_create and the set may not have been added to the
pool's list of sets yet.  While we're here, we move adding it to that
list into set_create for symmetry.

Fixes: 105002bd2d "anv: destroy descriptor sets when pool gets..."
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>