drm/nouveau/gr/gf100: Fix missing unlock in gf100_gr_chan_new()

When the call to gf100_grctx_generate() fails, unlock gr->fecs.mutex
before returning the error.

Fixes smatch warning:

drivers/gpu/drm/nouveau/nvkm/engine/gr/gf100.c:480 gf100_gr_chan_new() warn: inconsistent returns '&gr->fecs.mutex'.

Fixes: ca081ff ("drm/nouveau/gr/gf100-: generate golden context during first object alloc")
Signed-off-by: Li Huafei <lihuafei1@huawei.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241026173844.2392679-1-lihuafei1@huawei.com

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

@@ -443,6 +443,7 @@ gf100_gr_chan_new(struct nvkm_gr *base, struct nvkm_chan *fifoch,
 		ret = gf100_grctx_generate(gr, chan, fifoch->inst);
 		if (ret) {
 			nvkm_error(&base->engine.subdev, "failed to construct context\n");
+			mutex_unlock(&gr->fecs.mutex);
 			return ret;
 		}
 	}