drm/nouveau: fix null pointer dereference in nouveau_connector_get_modes

In the Linux kernel, the following vulnerability has been resolved:

drm/nouveau: fix null pointer dereference in nouveau_connector_get_modes

In nouveau_connector_get_modes(), the return value of drm_mode_duplicate()

is assigned to mode, which will lead to a possible NULL pointer

dereference on failure of drm_mode_duplicate(). Add a check to avoid npd.