xen/drm: switch xen_drm_front to use INVALID_GRANT_REF

Instead of using a private macro for an invalid grant reference use
the common one.
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Tested-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com> # Arm64 only
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/gpu/drm/xen/xen_drm_front.h

@@ -80,15 +80,6 @@ struct drm_pending_vblank_event;
 /* timeout in ms to wait for backend to respond */
 #define XEN_DRM_FRONT_WAIT_BACK_MS	3000
 
-#ifndef GRANT_INVALID_REF
-/*
- * Note on usage of grant reference 0 as invalid grant reference:
- * grant reference 0 is valid, but never exposed to a PV driver,
- * because of the fact it is already in use/reserved by the PV console.
- */
-#define GRANT_INVALID_REF	0
-#endif
-
 struct xen_drm_front_info {
 	struct xenbus_device *xb_dev;
 	struct xen_drm_front_drm_info *drm_info;

drivers/gpu/drm/xen/xen_drm_front_evtchnl.c

@@ -147,7 +147,7 @@ static void evtchnl_free(struct xen_drm_front_info *front_info,
 		xenbus_free_evtchn(front_info->xb_dev, evtchnl->port);
 
 	/* end access and free the page */
-	if (evtchnl->gref != GRANT_INVALID_REF)
+	if (evtchnl->gref != INVALID_GRANT_REF)
 		gnttab_end_foreign_access(evtchnl->gref, page);
 
 	memset(evtchnl, 0, sizeof(*evtchnl));
@@ -168,7 +168,7 @@ static int evtchnl_alloc(struct xen_drm_front_info *front_info, int index,
 	evtchnl->index = index;
 	evtchnl->front_info = front_info;
 	evtchnl->state = EVTCHNL_STATE_DISCONNECTED;
-	evtchnl->gref = GRANT_INVALID_REF;
+	evtchnl->gref = INVALID_GRANT_REF;
 
 	page = get_zeroed_page(GFP_NOIO | __GFP_HIGH);
 	if (!page) {