drm/gma500: fix null pointer dereference in psb_intel_lvds_get_modes

In psb_intel_lvds_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: 89c78134 ("gma500: Add Poulsbo support")
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/20240709092011.3204970-1-make24@iscas.ac.cn