drm/simple-helpers: Add missing includes

Add missing includes to pull in definitions for drm_crtc,
drm_plane and drm_encoder.
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170122181117.8210-4-noralf@tronnes.org

include/drm/drm_simple_kms_helper.h

@@ -10,6 +10,10 @@
 #ifndef __LINUX_DRM_SIMPLE_KMS_HELPER_H
 #define __LINUX_DRM_SIMPLE_KMS_HELPER_H
 
+#include <drm/drm_crtc.h>
+#include <drm/drm_encoder.h>
+#include <drm/drm_plane.h>
+
 struct drm_simple_display_pipe;
 
 /**