drm: drm_debugfs.h self-contained

While removing drmP.h from drm/radeon a few files ended
up including drm_debugfs.h as the first file.
This failed build due to missing dependencies in drm_debugfs.h.

Add the missing include files.

Signed-off-by: Sam Ravnborg <[e-mail]>
Reviewed-by: Alex Deucher <[e-mail]>
Cc: Maarten Lankhorst <[e-mail]>
Cc: Maxime Ripard <[e-mail]>
Cc: Sean Paul <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

include/drm/drm_debugfs.h

···
 #ifndef _DRM_DEBUGFS_H_
 #define _DRM_DEBUGFS_H_
 
+#include <linux/types.h>
+#include <linux/seq_file.h>
 /**
  * struct drm_info_list - debugfs info list entry
  *