drm/amd/display: check if ext_caps is valid in BL setup

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: check if ext_caps is valid in BL setup LVDS connectors don't have extended backlight caps so check if the pointer is valid before accessing it. (cherry picked from commit 3f797396d7f4eb9bb6eded184bbc6f033628a6f6)