drm/i915/dg2: Use I915_BO_ALLOC_CONTIGUOUS flag for DPT

Do mapping using CONTIGUOUS flag - otherwise
i915_gem_object_is_contiguous warn is triggered.

Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211209141817.16038-1-stanislav.lisovskiy@intel.com