drm/xe: Fix fence reservation accouting

Both execs and the preempt rebind worker can issue rebinds. Rebinds
require a fence, per tile, inserted into dma-resv slots of the VM and
BO (if external). The fence reservation accouting did not take into
account the number of fences required for rebinds, fix this.

v2: Rebase

Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reported-by: Christopher Snowhill <kode54@gmail.com>
Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/518


Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>