config: enable CONFIG_NOUVEAU_LEGACY_CTX_SUPPORT by default

hulk inclusion
category: bugfix
bugzilla: 4979
CVE: NA

-------------------------------------------------

Introduced by b30a43ac ("drm/nouveau: add kconfig
option to turn off nouveau legacy contexts. (v3)")
Enable CONFIG_NOUVEAU_LEGACY_CTX_SUPPORT by default

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>