drm/fb-helper: Set/clear dev->fb_helper in dummy init/fini

Set dev->fb_helper even when fbdev emulation is compiled out,
so drivers can use it to free the structure.
Clear it for consistency.

Fixes: 29ad20b ("drm: Add drm_device->fb_helper pointer")
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171215175119.36181-2-noralf@tronnes.org

include/drm/drm_fb_helper.h

@@ -33,6 +33,7 @@
 struct drm_fb_helper;
 
 #include <drm/drm_crtc.h>
+#include <drm/drm_device.h>
 #include <linux/kgdb.h>
 
 enum mode_set_atomic {
@@ -332,11 +333,17 @@ static inline int drm_fb_helper_init(struct drm_device *dev,
 		       struct drm_fb_helper *helper,
 		       int max_conn)
 {
+	/* So drivers can use it to free the struct */
+	helper->dev = dev;
+	dev->fb_helper = helper;
+
 	return 0;
 }
 
 static inline void drm_fb_helper_fini(struct drm_fb_helper *helper)
 {
+	if (helper && helper->dev)
+		helper->dev->fb_helper = NULL;
 }
 
 static inline int drm_fb_helper_blank(int blank, struct fb_info *info)