i965/batch: avoid reverting batch buffer if saved state is an empty

i965/batch: avoid reverting batch buffer if saved state is an empty

There's no point reverting to the last saved point if that save point is
the empty batch, we will just repeat ourselves.

CC: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: 3faf56ffbdeb "intel: Add an interface for saving/restoring
                     the batchbuffer state."
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107626
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>