drm/i915/display: Use intel_bo_to_drm_bo instead of obj->base

Xe and i915 objects have differing implementation. Use intel_bo_to_drm_bo
instead of obj->base as xe_bo doesn't have base pointer.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231012072158.4115795-3-jouni.hogander@intel.com