agp/intel: Destroy the scatterlist on allocation failure

A side-effect of being able to use custom page allocations with the
sg_table is that it cannot reap the partially constructed scatterlist if
fails to allocate a page. So we need to call sg_free_table() ourselves
if sg_alloc_table() fails.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc Dave Airlie <airlied@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>