drm: fix implicit declaration build error on ia64

   drivers/gpu/drm/drm_dp_helper.c: In function 'drm_dp_downstream_debug':
>> drivers/gpu/drm/drm_dp_helper.c:551:2: error: implicit declaration of
>> function 'seq_printf' [-Werror=implicit-function-declaration]
     seq_printf(m, "\tDP branch device present: %s\n",
     ^
>> drivers/gpu/drm/drm_dp_helper.c:559:3: error: implicit declaration of
>> function 'seq_puts' [-Werror=implicit-function-declaration]
     seq_puts(m, "\t\tType: DisplayPort\n");
     ^
Reported-by: kbuild test robot <fengguang.wu@intel.com>
References: https://lists.freedesktop.org/archives/intel-gfx/2016-September/106638.html
Fixes: 80209e5f ("drm: Add DP branch device info on debugfs")
Cc: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1474020396-14875-1-git-send-email-jani.nikula@intel.com

drivers/gpu/drm/drm_dp_helper.c

@@ -27,6 +27,7 @@
 #include <linux/errno.h>
 #include <linux/sched.h>
 #include <linux/i2c.h>
+#include <linux/seq_file.h>
 #include <drm/drm_dp_helper.h>
 #include <drm/drmP.h>