drm/arcpgu: Use drm_atomic_helper_shutdown

drm_plane_helper_disable is a non-atomic drivers only function, and
will blow up (since no one passes the locking context it needs).

Atomic drivers which want to quiescent their hw on unload should
use drm_atomic_helper_shutdown() instead.
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004202446.22905-11-daniel.vetter@ffwll.ch

drivers/gpu/drm/arc/arcpgu_crtc.c

@@ -184,7 +184,6 @@ static const struct drm_plane_helper_funcs arc_pgu_plane_helper_funcs = {
 
 static void arc_pgu_plane_destroy(struct drm_plane *plane)
 {
-	drm_plane_helper_disable(plane, NULL);
 	drm_plane_cleanup(plane);
 }
 

drivers/gpu/drm/arc/arcpgu_drv.c

@@ -134,6 +134,7 @@ static int arcpgu_unload(struct drm_device *drm)
 		arcpgu->fbdev = NULL;
 	}
 	drm_kms_helper_poll_fini(drm);
+	drm_atomic_helper_shutdown(drm);
 	drm_mode_config_cleanup(drm);
 
 	return 0;