drm/amd/display: Check null pointers before using them

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Check null pointers before using them [WHAT & HOW] These pointers are null checked previously in the same function, indicating they might be null as reported by Coverity. As a result, they need to be checked when used again. This fixes 3 FORWARD_NULL issue reported by Coverity.