drm/nouveau/mmu/r535: uninitialized variable in r535_bar_new_()

If gf100_bar_new_() fails then "bar" is not initialized.

Fixes: 5bf02571 ("drm/nouveau/mmu/r535: initial support")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/dab21df7-4d90-4479-97d8-97e5d228c714@moroto.mountain