drm/nouveau/dispnv04: fix null pointer dereference in nv17_tv_get_ld_modes
In the Linux kernel, the following vulnerability has been resolved:

drm/nouveau/dispnv04: fix null pointer dereference in nv17_tv_get_ld_modes

In nv17_tv_get_ld_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.