anv/batch: Move last_ss_pool_bo_offset to the command buffer

anv/batch: Move last_ss_pool_bo_offset to the command buffer

The original reason for putting it in the batch_bo was to allow primaries
to share it across secondaries or something like that.  However, the
relocation lists in secondary command buffers are are always left alone and
copied into the primary command buffer's relocation list.  This means that
the offset really applies at the command buffer level and putting it in the
batch_bo doesn't make sense.  This fixes a couple of potential bugs around
re-submission of command buffers that are not likely to be hit but are bugs
none the less.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Cc: "13.0" <mesa-stable@lists.freedesktop.org>