drm/i915/display: In intel_framebuffer_init switch to use intel_bo_to_drm_bo

We are preparing for Xe driver. I915 and Xe object implementation are
differing. Use intel_bo_to_drm_bo instead of &obj->base.

Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231116150225.204233-3-juhapekka.heikkila@gmail.com