drm/i915: Protect against deferred fbdev setup

We could probably hit this already with our current async fbdev init,
but it's much easier to hit this with the new deferred fbdev setup
that I'm working on polishing.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reported-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170706130023.28417-2-daniel.vetter@ffwll.ch

drivers/gpu/drm/i915/i915_debugfs.c

@@ -1935,7 +1935,7 @@ static int i915_gem_framebuffer_info(struct seq_file *m, void *data)
 		return ret;
 
 #ifdef CONFIG_DRM_FBDEV_EMULATION
-	if (dev_priv->fbdev) {
+	if (dev_priv->fbdev && dev_priv->fbdev->helper.fb) {
 		fbdev_fb = to_intel_framebuffer(dev_priv->fbdev->helper.fb);
 
 		seq_printf(m, "fbcon size: %d x %d, depth %d, %d bpp, modifier 0x%llx, refcount %d, obj ",