It doesn't even show up in any header files and only used iternally.
Originally it was (ab)used to restore the fbcon on lastclose, but that
died with
commit 
e8e7a2b8ccfdae0d4cb6bd25824bbedcd42da316
Author: Dave Airlie <airlied@redhat.com>
Date:   Thu Apr 21 22:18:32 2011 +0100
    drm/i915: restore only the mode of this driver on lastclose (v2)
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
 
        return error;
 }
 
-int drm_fb_helper_panic(struct notifier_block *n, unsigned long ununsed,
+static int drm_fb_helper_panic(struct notifier_block *n, unsigned long ununsed,
                        void *panic_str)
 {
        /*
        pr_err("panic occurred, switching back to text console\n");
        return drm_fb_helper_force_kernel_mode();
 }
-EXPORT_SYMBOL(drm_fb_helper_panic);
 
 static struct notifier_block paniced = {
        .notifier_call = drm_fb_helper_panic,