drm/gma500: fix null pointer dereference in cdv_intel_lvds_get_modes

In cdv_intel_lvds_get_modes(), the return value of drm_mode_duplicate()
is assigned to mode, which will lead to a NULL pointer dereference on
failure of drm_mode_duplicate(). Add a check to avoid npd.

Cc: stable@vger.kernel.org
Fixes: 6a227d5f ("gma500: Add support for Cedarview")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240709113311.37168-1-make24@iscas.ac.cn