drm/nouveau/disp: switch to use kmemdup() helper

Use kmemdup() helper instead of open-coding to
simplify the code.

Signed-off-by: Chen Haonan <chen.haonan2@zte.com.cn>
Reviewed-by: Yang Guang <yang.guang5@zte.com.cn>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/202401091424115185126@zte.com.cn