drm/mst: fix build with debugfs off.

seq_file.h was being pulled in via drm_mm.h in that case.

Reported-by: Matthew Thode (on irc)
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_dp_mst_topology.c

@@ -25,6 +25,7 @@
 #include <linux/init.h>
 #include <linux/errno.h>
 #include <linux/sched.h>
+#include <linux/seq_file.h>
 #include <linux/i2c.h>
 #include <drm/drm_dp_mst_helper.h>
 #include <drm/drmP.h>