drm/xe/exec: reserve fence slot for CPU bind

Looks possible to switch from CPU binding to GPU binding mid exec, and
if that happens for the same dma-resv we might use two fence slots, once
for the dummy fence, and another for the actual GPU bind.

References: drm/xe/kernel#698


Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>