anv: fix descriptor pool internal size allocation

anv: fix descriptor pool internal size allocation

The size of the pool is slightly smaller than the size of the
structure containing the whole pool. We need to take that into account
on when setting up the internals.

Fixes a crash due to out of bound memory access in:
   dEQP-VK.api.descriptor_pool.out_of_pool_memory

v2: Drop debug traces (Lionel)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Cc: "17.0 13.0" <mesa-stable@lists.freedesktop.org>