drm/nouveau/gr/gf100-: fix oops when calling zbc methods

Somehow missed these two when removing dodgy void casts during the
rework.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nvkm/engine/gr/gf100.c

@@ -143,7 +143,7 @@ gf100_gr_zbc_depth_get(struct gf100_gr *gr, int format,
 static int
 gf100_fermi_mthd_zbc_color(struct nvkm_object *object, void *data, u32 size)
 {
-	struct gf100_gr *gr = (void *)object->engine;
+	struct gf100_gr *gr = gf100_gr(nvkm_gr(object->engine));
 	union {
 		struct fermi_a_zbc_color_v0 v0;
 	} *args = data;
@@ -189,7 +189,7 @@ gf100_fermi_mthd_zbc_color(struct nvkm_object *object, void *data, u32 size)
 static int
 gf100_fermi_mthd_zbc_depth(struct nvkm_object *object, void *data, u32 size)
 {
-	struct gf100_gr *gr = (void *)object->engine;
+	struct gf100_gr *gr = gf100_gr(nvkm_gr(object->engine));
 	union {
 		struct fermi_a_zbc_depth_v0 v0;
 	} *args = data;