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

drivers/gpu/drm/i915/display/intel_fbdev.c

@@ -332,12 +332,12 @@ static bool intel_fbdev_init_bios(struct drm_device *dev,
 			continue;
 		}
 
-		if (obj->base.size > max_size) {
+		if (intel_bo_to_drm_bo(obj)->size > max_size) {
 			drm_dbg_kms(&i915->drm,
 				    "found possible fb from [PLANE:%d:%s]\n",
 				    plane->base.base.id, plane->base.name);
 			fb = to_intel_framebuffer(plane_state->uapi.fb);
-			max_size = obj->base.size;
+			max_size = intel_bo_to_drm_bo(obj)->size;
 		}
 	}