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

We are preparing for Xe. Xe_bo doesn't have obj->base. Due to this
use intel_bo_to_drm_bo instead in intel_prepare_plane_fb.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231102112219.1039362-1-jouni.hogander@intel.com