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.

Cc: stable@vger.kernel.org
Fixes: cf13909a ("drm/fb-helper: Move out modeset config code")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240802044736.1570345-1-make24@iscas.ac.cn