drm/nouveau/dispnv04: fix null pointer dereference in nv17_tv_get_hd_modes
In the Linux kernel, the following vulnerability has been resolved:

drm/nouveau/dispnv04: fix null pointer dereference in nv17_tv_get_hd_modes

In nv17_tv_get_hd_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(). The same applies to drm_cvt_mode().
Add a check to avoid null pointer dereference.