drm/xe/migrate: use XE_BO_FLAG_PAGETABLE

On some HW we want to avoid the host caching PTEs, since access from GPU
side can be incoherent. However here the special migrate object is
mapping PTEs which are written from the host and potentially cached. Use
XE_BO_FLAG_PAGETABLE to ensure that non-cached mapping is used, on
platforms where this matters.

Fixes: 7a060d78 ("drm/xe/mtl: Map PPGTT as CPU:WC")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Cc: <stable@vger.kernel.org> # v6.8+
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241126181259.159713-4-matthew.auld@intel.com