drm/i915/display: use intel_bo_to_drm_bo in intel_fbdev

We are preparing for Xe driver. I915 and Xe object implementation are
differing. Do not use  i915_gem_object->base directly. Instead use
intel_bo_to_drm_bo.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231115090719.3210079-3-jouni.hogander@intel.com