drm/nouveau/svm: remove unused but set variables

Fix the W=1 warning -Wunused-but-set-variable.

Cc: Karol Herbst <kherbst@redhat.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Danilo Krummrich <dakr@redhat.com>
Cc: nouveau@lists.freedesktop.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8b133e7ec0e9aef728be301ac019c5ddcb3bbf51.1704908087.git.jani.nikula@intel.com