drm/nouveau: fix for disabled fbdev emulation

Hello,

after this commit:

commit f045f459d925138fe7d6193a8c86406bda7e49da
Author: Ben Skeggs <bskeggs@redhat.com>
Date:   Thu Jun 2 12:23:31 2016 +1000
    drm/nouveau/fbcon: fix out-of-bounds memory accesses

kernel started to oops when loading nouveau module when using GTX 780 Ti
video adapter. This patch fixes the problem.

Bug report: https://bugzilla.kernel.org/show_bug.cgi?id=120591

Signed-off-by: Dmitrii Tcvetkov <demfloro@demfloro.ru>
Suggested-by: Ilia Mirkin <imirkin@alum.mit.edu>
Fixes: f045f459d925 ("nouveau_fbcon_init()")
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/nouveau/nouveau_fbcon.c

@@ -552,7 +552,8 @@ nouveau_fbcon_init(struct drm_device *dev)
 	if (ret)
 		goto fini;
 
-	fbcon->helper.fbdev->pixmap.buf_align = 4;
+	if (fbcon->helper.fbdev)
+		fbcon->helper.fbdev->pixmap.buf_align = 4;
 	return 0;
 
 fini: