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

Vulnerability Description

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.

References

• https://git.kernel.org/stable/c/26787fb6c2b2ee0d1a7e1574b36f4711ae40fe27
• https://git.kernel.org/stable/c/fdd5ecbbff751c3b9061d8ebb08e5c96119915b4