drm/i915: fix ppgtt dump code for DEBUG_FS=n

A regression in the topic/ppgtt branch introduce in

commit 87d60b6
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Fri Dec 6 14:11:29 2013 -0800

    drm/i915: Add PPGTT dumper

The issue is that we're missing the definitions for the seq_file
functions and hence compilation fails.

v2: Just include the right header instead of splattering #ifdefs all
over the place (Chris).

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reported-by: Antti Koskipaa <antti.koskipaa@linux.intel.com>
Cc: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -22,6 +22,7 @@
  *
  */
 
+#include <linux/seq_file.h>
 #include <drm/drmP.h>
 #include <drm/i915_drm.h>
 #include "i915_drv.h"