drm/amd/display: Check null pointers before multiple uses

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Check null pointers before multiple uses [WHAT & HOW] Poniters, such as stream_enc and dc->bw_vbios, are null checked previously in the same function, so Coverity warns "implies that stream_enc and dc->bw_vbios might be null". They are used multiple times in the subsequent code and need to be checked. This fixes 10 FORWARD_NULL issues reported by Coverity.