drm: mxsfb: fix fence synchronization

The conversion away from the simple display pipeline helper missed
to convert the prepare_fb plane callback, so no fences are attached to
the atomic state, breaking synchronization with other devices. Fix
this by plugging in the drm_gem_fb_prepare_fb helper function.

Fixes: ae1ed009 ("drm: mxsfb: Stop using DRM simple display pipeline helper")
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201120211306.325841-1-l.stach@pengutronix.de

drivers/gpu/drm/mxsfb/mxsfb_kms.c

@@ -22,6 +22,7 @@
 #include <drm/drm_fb_cma_helper.h>
 #include <drm/drm_fourcc.h>
 #include <drm/drm_gem_cma_helper.h>
+#include <drm/drm_gem_framebuffer_helper.h>
 #include <drm/drm_plane.h>
 #include <drm/drm_plane_helper.h>
 #include <drm/drm_vblank.h>
@@ -485,11 +486,13 @@ static void mxsfb_plane_overlay_atomic_update(struct drm_plane *plane,
 }
 
 static const struct drm_plane_helper_funcs mxsfb_plane_primary_helper_funcs = {
+	.prepare_fb = drm_gem_fb_prepare_fb,
 	.atomic_check = mxsfb_plane_atomic_check,
 	.atomic_update = mxsfb_plane_primary_atomic_update,
 };
 
 static const struct drm_plane_helper_funcs mxsfb_plane_overlay_helper_funcs = {
+	.prepare_fb = drm_gem_fb_prepare_fb,
 	.atomic_check = mxsfb_plane_atomic_check,
 	.atomic_update = mxsfb_plane_overlay_atomic_update,
 };