anv: Fix pool allocator when first alloc needs to grow

When using softpin, the first allocation was not calculating the
padding and offset correctly for the case the first allocation needed
to grow.  We were missing initialize the state.end right after
expanding the pool for the first time.

This is not a problem for non-softpin since there we don't use
leftover padding so the ends would re-arrange incrementally.

This fixes running dEQP-VK.ssbo.phys.layout.random.16bit.scalar.13 in
SKL -- the test uses a shader larger than the initial size for the
instruction pool.

Fixes: dfc9ab2c "anv/allocator: Add padding information."
Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 09c4037d)