drm/client: fix null pointer dereference in drm_client_modeset_probe

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

drm/client: fix null pointer dereference in drm_client_modeset_probe

In drm_client_modeset_probe(), the return value of drm_mode_duplicate() is

assigned to modeset->mode, which will lead to a possible NULL pointer

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