drm/i915/display: in skl_surf_address check for dpt-vma

touch dpt_vma->node only if dpt-vma is not NULL

Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231116150225.204233-2-juhapekka.heikkila@gmail.com