drm/xe/vm: Prefer xe_assert() over XE_WARN_ON()

When xelp_pte_encode_addr() was added in commit 23c8495e


("drm/xe/migrate: Do not hand-encode pte"), there was no xe pointer for
using xe_assert(). This is not the case anymore, so prefer it over
XE_WARN_ON().

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>