drm/panel/panel-tpo-tpg110: fix a possible null pointer dereference

drm/panel/panel-tpo-tpg110: fix a possible null pointer dereference

In tpg110_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.

Signed-off-by: Ma Ke <make_ruc2021@163.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20231009090446.4043798-1-make_ruc2021@163.com
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231009090446.4043798-1-make_ruc2021@163.com