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.

Cc: stable@vger.kernel.org
Fixes: 6ee73861 ("drm/nouveau: Add DRM driver for NVIDIA GPUs")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240627074204.3023776-1-make24@iscas.ac.cn