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 <[email protected]>
References: https://lists.freedesktop.org/archives/intel-gfx/2016-September/106638.html
Fixes: 80209e5 ("drm: Add DP branch device info on debugfs")
Cc: Mika Kahola <[email protected]>
Signed-off-by: Jani Nikula <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]

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>